Rotary Impact Tool And Clutch Therefor

Abstract

This invention pertains to a rotary impact tool and a clutch therefor, wherein a motor drives a cage member within which is pivotally mounted a swinging hollow hammer member. An output shaft extends through the cage member and through the hollow hammer member and includes forward and reverse impact anvil surfaces. The hammer member is mounted to swing in respect to the cage as it rotates with the cage, and it carries forward and reverse impact jaws on its internal surface. As the clutch is driven in the forward direction by an air motor or the like, the forward impact jaw is moved in and out of the path of the anvil jaw on the output shaft by cam action and during an impact blow the inertia of the rotating hammer member acts as automatic means to hold the impact jaw in engagement with the anvil jaw. A second embodiment includes a pair of hammers arranged to simultaneously strike a pair of anvil jaws on the anvil.

Parent Case Text

This application is a continuation-in-part of my copending application, Ser. No. 852,574, filed Aug. 25, 1969, for a Rotary Impact Tool and Clutch Therefor, now abandoned.

Description

This invention relates to a rotary impact wrench and clutch therefor and more particularly to an air driven impact wrench.

A conventional rotary impact wrench mechanism is known as a "swinging weight" mechanism and is disclosed in U.S. Pat. No. 2,285,638, issued to L. A. Amtsberg. This mechanism uses a pair of diametrically opposed tilting hammer dogs or members which rotate around a lobed anvil and are cammed into an impact position with the lobes or jaws on the anvil by engagement with the anvil. The hammer dogs are released by the cams on the anvil immediately before impact and means is provided for applying a drive torque to the dogs to cause them to rotate to a disengaging position following an impact.

This version of "swinging weight" type of mechanism shown in the foregoing patent is believed to have certain disadvantages and one of these is that it often rebounds after impact and strikes a second blow before disengaging the hammer from the anvil for continued rotation. Also, this mechanism is believed to be inefficient in delivering its blow energy to the anvil because a portion of such energy is used to disengage the hammer member, causing such member to tilt toward disengaging position during the impact.

Another version of the "swinging weight" type of mechanism is shown in U.S. Pat. No. 2,580,631 to E. R. Whitledge. In this version, the anvil carries a pair of axially and diametrically spaced lobes or jaws and a pair of axially spaced hammer members are pivoted in a hammer carrier on diametrically located pins with each hammer member being extended around the anvil. It is believed that this version also has problems of striking a second blow following impact and of using a part of the impact energy to cam the hammer member into disengaging position. In addition, in this later version, due to the location of the impact surface in the hammer member following or lagging its pivot, the impact creates tensional stresses in the hammer member which are less desirable than compressive stresses.

The principal object of this invention is to provide a novel impact mechanism which either eliminates or substantially minimizes the foregoing problems and is a more efficient impact mechanism.

Another object of this invention is to provide an impact tool and clutch combination which results in a low cost, efficient, durable tool which is light in weight, powerful in its impacting action, and which has good run-down characteristics.

A further object of the invention is to provide an impact clutch with a minimum number of movable parts which are easily and inexpensively formed, resulting in a low cost, reliable, and durable impact tool.

Another object of the invention is to provide a clutch for an impact tool which is capable of efficient operation at both low and high output torques.

Further important objects include the following: to provide a "swinging weight" impact wrench mechanism having a hammer member which is substantially free of tensional stresses during impact; to provide a "swinging weight" impact wrench mechanism having a swinging hammer pivoted on a novel type of pivot; to provide a "swinging weight" mechanism of simplified construction which prevents the hammer from tilting toward a disengaged position during rebound following impact; to provide a more efficient "swinging weight" type of impact wrench mechanism; to provide a "swinging weight" mechanism that is held by centrifugal force in anvil engaging position prior to impact; to provide a "swinging weight" mechanism having the center of mass of the hammer near the center of rotation of the mechanism; and to provide a "swinging weight" mechanism that strikes a balanced blow to the anvil.

An aspect of the present invention lies in the provision of a rotary impact tool having a housing within which a motor is mounted. The output shaft of the tool is mounted on the housing for rotation and it includes an impact receiving anvil jaw generally radially disposed on its periphery. A hollow cage or carrier member is coaxially around the output shaft and is mounted for rotation in respect to the tool output shaft. A rigid driving connection exists between the motor and the cage member so that the cage member rotates with the motor. A hollow hammer member is pivotally connected in the cage member for rotation therewith as the motor drives the cage member and for angular pivotal motion relative to the cage member about an axis offset from but parallel to the axis of rotation of the cage member. The hammer member has an impact delivering jaw on its inside surface located between the axes and positioned to always lead its pivotal connection. The impact jaw is movable into and out of the path of the impact receiving anvil to deliver an impact blow thereto. Cam means cause the angular movement of the impact delivering jaw into the path of the anvil jaw where it is held by centrifugal force until impact, and the inertia of the rotating hammer member acts to prevent the disengagement during the impact blow. Automatic means cause angular movement of the impact jaw out of the path of the anvil jaw at the end of the impact blow. The carrier can contain two hammers for simultaneously striking a pair of anvil jaws to deliver a balanced impact torque to the output shaft.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

With reference to the drawings:

FIG. 1 is a side view of an impact tool showing the clutch portion in longitudinal section;

FIGS. 2, 3, 4, and 5 are sectional views taken along line A--A of FIG. 1, with FIG. 2 showing the clutch in its impact position;

FIG. 3 shows the clutch in its disengaged position;

FIG. 4 shows the clutch in position to start cam engagement;

FIG. 5 shows the position of the parts at the end of the cam engagement;

FIG. 6 is a longitudinal section of a second embodiment of the invention;

FIG. 7 is an enlarged section taken on line 7--7 in FIG. 6 showing the hammer in impact position; and

FIGS. 8 and 9 are sections similar to FIG. 7 on a smaller scale showing the position of the hammer following an impact.

With reference to the drawings, and particularly to the first embodiment 1 shown in FIGS. 1 to 5, reference character 10 identifies the housing for an air driven impact wrench, the air motor of which is well known in the art and need not be described in detail.

The output shaft 11 of the air motor is coupled through meshing splines 12, 13 to a hollow cage or carrier member 15 which is journaled by sleeve bearing 17 on the tool power output shaft 19. The motor shaft 11 is coaxially aligned with the power output shaft 19 and the cage member 15 is coaxially mounted around the output shaft 19, and is mounted for rotating in respect to the output shaft 19. The cage member 15 comprises a pair of longitudinally spaced end plates 14 joined by a pair of diametrically spaced longitudinally extending struts 16 joining together the end plates. The rear end portion of the output shaft or spindle 19 is integrally formed as an anvil carrying an anvil jaw 23 extending generally radially therefrom and providing a forward impact receiving surface 20 and a reverse impact receiving surface 21. The forward end of output shaft 19 is carried by bushing 9 mounted in the forward end of tool housing 10.

Within the internal diameter of the hollow cage member 15 there is a channel 18 along one of the struts 16 in which is positioned a pivot 22, forming, in effect, a roller and socket connection. The pivot 22 is a portion of a hollow hammer member or dog 25 which is mounted around the output shaft 19. Thus the hammer member 25 is pivotally connected in the cage member 15 about a tilt axis formed by the pivot 22 so that it rotates with the cage member under drive from the motor output shaft 11, and additionally can move with an angular pivot motion, relative to the cage member 15, about the tilt axis offset from, but parallel to, the axis of rotation of the cage member.

The hollow hammer member 25 has on its internal surface 26 a forward impact jaw or surface 27 and a reverse impact jaw or surface 28 which are movable into and out of the path of the impact receiving surfaces 20, 21 respectively, as the tool operates in the forward or reverse direction. The hammer 25 is shaped in cross-section or symmetrical halves joined along a plane extending through the axis 22.

The operation of the mechanism is explained starting from the moment of impact which is shown in FIG. 2, with the forward impact jaw 27 of the cage member 15 in a hammer blow engagement with the forward anvil surface 20 of the output shaft 19. The motor output shaft 11 is directly driving cage 15 in a clockwise direction as shown by the arrow in FIG. 3. Immediately following the impact, the hammer member 25 tilts in a counterclockwise direction about pivot 22 until the jaws disengage as shown in FIG. 3. This tilting movement of the hammer member 25 is caused either by inertia forces during rebound of the hammer member 25 following impact or by the motor torque driving the hammer member against the anvil impact surface 20 which cams the hammer member counterclockwise. This automatic disengagement of the hammer member 25 will be more fully explained later.

The cage 15 and hammer member 25 are now free from the anvil jaw 23 and accelerate in unison in a clockwise direction about the center axis of the anvil until the position shown in FIG. 4 is reached, at which position cam engagement is about to commence. Continued forward rotation of the hammer member 25 causes the reverse impact jaw 28 on the hammer member 25 to ride up over the forward anvil impact surface 20 on the anvil jaw 23, which cams the hammer 25 back to its original, or engaged position, where it is maintained by centrifugal force acting on the center of gravity of the hammer member 25.

Continued rotation of the cage 15 and hammer 25 in unison, shown by FIG. 5, brings the parts back to their original positions and another impact blow is delivered. During a blow the inertia of the rotating hammer member acting on the anvil acts to prevent disengagement of the hammer until the momentum of both the cage and hammer member has been expended.

An advantage of this tool lies in the fact that the total kinetic energy of the motor rotor from motor shaft 11, the cage 15 and the hammer member 25 is used in each impact, since there can be no disengaging action until the momentum of the hammer member has been dissipated. The disengaging torque produced by the momentum of the rotor and cage is countered by the engaging torque created by deceleration of the hammer member. When the momentum of the rotating parts has been dissipated, disengagement can occur under the influence of the motor torque, and the cycle begins again. This action is further explained later.

When a nut is loose the tool acts to run it down without impacting until sufficient resistance is encountered, at which point the tool automatically commences to impact. During run-down the clutch parts are in the position shown in FIG. 2, and due to centrifugal force and friction between the hammer and anvil jaws good run-down torque is obtained from the motor, directly through the cage member 15 to the hammer member 25, and thence directly to the tool output shaft 19.

In forward rotation the forward impact jaw 27 always leads the pivot point 22 during rotation so that compressive stress is set up in the hammer member between the jaw and the pivot point during an impact blow. During reverse action the same effect is achieved in reverse direction.

During reverse, or loosening action of the tool, the hammer member 25 is in impacting position, similar to FIG. 2, but with reverse impact jaw 28 against reverse anvil surface 21. The impacting action is similar to the forward impacting action.

The second embodiment 30 shown in FIGS. 6 to 9 strikes a pair of simultaneous impacts on the anvil at diametrically spaced points to deliver a balanced impact torque to the anvil. The carrier or cage 15 carries a pair of identically shaped hammer members or dogs 25 and 25' spaced longitudinally in the cage 15 and pivoted on tilting axes spaced diametrically from each other. The anvil shaft 19 carries a pair of anvil jaws 23 and 23' located longitudinally and diametrically from each other.

Each hammer member 25, 25' pivots on a longitudinal pin 32, 32' providing tilting axes and acting similar to the roller and socket pivot 22 in FIG. 2. In this second embodiment, the hammer member 25 carries a longitudinal groove seating on its pivot pin 32 and the pivot pin 32 fits in a longitudinal internal channel extending the length of a strut 16 of the cage 15. This arrangement is desirable since the pin 32 is not subjected to shearing stresses and the hammer member 25 avoids tensional stresses which would be present if the pin 32 extended through a closed pivot bore in the hammer member 25. The above described is also true for the hammer member 25' and pin 32'.

Each hammer member 25, 25' engages a stop means to limit its tilting movement in each direction about its pin 32, 32'. The hammer member 25 includes a notch 33 diametrically opposite its tilt pin 32 fitting over the opposite pin 32' and the angular extent of the notch 33 is sized to abut the pin 32' as shown in FIGS. 7 and 8 to limit the tilting movement of the hammer member 25 and prevent the hammer jaws from banging against the anvil at the ends of its tilting movements. The hammer member 25' includes a similar notch 33' received over the pin 32 to serve in the same manner as a stop means.

It is believed to be worthwhile to explain the various forces acting on this impact mechanism at various stages of its operating cycle in order for the reader to appreciate the benefits provided by this mechanism over the prior art. Many of these forces are shown diagrammatically in FIG. 7, which shows a hammer member 25 of the second embodiment in impact position against the anvil jaw 23.

Prior to impact, the hammer member 25 is rotating in a clockwise direction as shown by the arrow X about the axis CR of the carrier 15 and the hammer member 25 is tilted about its tilt axis CT to offset its center of mass CM to the left of the rotation center CR. Due to the unbalance of the hammer member 25, a centrifugal force G is created acting along the dotted line 36 extending through the rotating center CR and the offset center of mass CM. This centrifugal force G holds the hammer member 25 in the engagement position shown in FIG. 7 so long as the carrier 15 rotates at a high speed.

When the hammer member 25 strikes the anvil jaw 23, it decelerates very rapidly causing several inertia forces to act on the hammer member 25. The impact surfaces 20 and 27 are formed to impact along the radial plane indicated by the dotted line 37. The plane is located to provide an impact force line 38 which extends normal to the plane 37 and is located a short distance to the right of the tilt axis CT. The force line 38 represents the direction of the impact forces delivered to the anvil. The force line 38 is located slightly outside of the tilt axis CT in order for the motor torque to be able to cam the hammer member 25 to a disengaged position, as shown in FIG. 9, under certain conditions of operation, for example, if the motor is started with the hammer member 25 at rest in the position shown in FIG. 7. The moment arm for the camming force B acting along the force line 38 about the tilt axis CT is shown as M and should be short in order that the camming torque is not too large. In other words, this camming action is necessary to prevent the mechanism from locking up under certain conditions but should be no greater than necessary to accomplish this purpose. As will be explained later, the hammer member 25 is normally tilted to a disengaging position by inertia forces.

During the instant of impact, while the hammer member 25 is decelerating and delivering its impact energy to the anvil, inertia forces caused by the deceleration act to overcome the camming force B and to hold the hammer member 25 against tilting counterclockwise to the disengaging position. This action is called "impact lock-up" and is necessary in order for the hammer member to deliver its full blow energy to the anvil.

During deceleration, inertia forces in the hammer include a linear motion force A acting through the center of mass CM and normal to the line 36 and towards the right to either oppose or reinforce the camming force B acting along the force line 38 depending upon the exact location of the center of mass CM. As shown in FIG. 7 the force A would be reinforce camming force B.

Another inertia force is also acting on the hammer member 25 due to its angular deceleration and this is a force couple T acting in a clockwise direction. The force couple T is combined with the linear force A to form a resultant inertia force F acting to the right, the normal to the line 36, through the center of percussion CP. The resultant force F is equal to the linear force A and is located much further from the rotation center CR, where it extends to the left of the tilt axis CT and has a moment arm L about the tilt axis CT. Due to its location, the resultant inertia force F applies a clockwise torque on the hammer member 25 about its tilt axis CT to overcome the camming force B acting along the force line 38. Thus, the hammer member 25 is prevented from moving during the instant of impact. It is believed that the moment arm L should be at least as long as the moment arm M for the camming force B.

Normally, the hammer member 25 and carrier 15 rebounds through a counterclockwise angle following impact due to the resilient nature of the mechanism much the same as a carpenter's hammer rebounds after a blow. The angle of travel of a rebounding hammer can be quite large, for example, as much as 120.degree. . During the rebounding travel, the motor is attempting to decelerate the hammer member 25 and this creates an inertia force R acting through the center of percussion CP normal to the line 36 and opposite to the "impact lock-up" force F. The force R applies a counterclockwise torque on the hammer member 25 causing it to swing rapidly counterclockwise to its disengaging position as shown in FIG. 8.

After the hammer member 25 reaches the disengaged position, the force R shifts to a new position and becomes the force H, shown in FIG. 8, which continues to hold the hammer member 25 in its disengaged position while the cage 15 is accelerated forward in the clockwise direction. This action ensures that the hammer member 25 does not strike the anvil jaw 23 a second blow before rotating past the anvil jaw 23.

It is recognized that the hammer 25 probably does not have to extend completely around the anvil to obtain all of the advantages disclosed for this mechanism. However, it is believed that the hammer should extend at least 180.degree. around the anvil and the center of mass of the hammer should be closer to the axis of rotation than to the tilt axis of the hammer.

While there have been described what is at present considered to be two preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Claims

What is claimed is:

1. A rotary impact tool comprising, in combination, a housing, a motor mounted in said housing, an output shaft mounted on said housing for rotation and including an impact receiving anvil jaw generally radially disposed on its periphery, a carrier member coaxially around said output shaft and mounted for rotation in respect to said output shaft, driving connection means between said motor and said carrier member for rotating said carrier member, a hammer member pivotally connected in said carrier member for rotation therewith and for angular pivotal motion relative thereto about an axis offset from but parallel to the axis of rotation of said carrier member, said hammer member for clockwise impact operation having a clockwise impact delivering jaw on its inside surface located between 0.degree. and 90.degree. clockwise from its pivot connection to the carrier member, said impact jaw being movable into and out of the path of said impact receiving anvil jaw to deliver an impact blow thereto, cam means for effecting the angular pivot movement of said impact delivering jaw into the path of said anvil jaw in a clockwise direction relative to said carrier member, centrifugal force created by the proportions, mass and mass center location of said hammer member holding said impact delivering jaw in the path of said anvil jaw until the delivery of said impact blow thereto, and automatic means for effecting the angular pivot movement of said impact jaw out of the path of said anvil jaw in a counter clockwise direction in relation to said carrier member after the delivery of said impact blow.

2. A rotary impact tool as set forth in claim 1 wherein said hammer member extends around said output shaft for at least 170.degree..

3. A rotary impact tool as set forth in claim 2 wherein said hammer member completely surrounds said output shaft.

4. A rotary impact tool comprising, in combination, a housing, a motor mounted in said housing, an output shaft mounted on said housing for rotation and including an impact receiving anvil jaw generally radially disposed on its periphery, a carrier member coaxially around said output shaft and journaled for rotation in respect to said output shaft, driving connection means between said motor and said carrier member for rotating said carrier member, a hammer member pivotally connected in said carrier member for rotation therewith and for angular pivotal motion relative thereto about an axis offset from but parallel to the axis of rotation of said carrier member, said hammer member having a clockwise impact jaw on its inner surface located between 0.degree. and 90.degree. clockwise from its pivot axis to the carrier member so that when the hammer member is pivoted in a clockwise direction the clockwise impact jaw moves inwardly toward said impact receiving anvil jaw, cam means to produce said pivot motion, and centrifugal detent means to hold said impact jaw in said pivoted position to mate with the anvil jaw and produce a disengaging pivot torque on the hammer member tending to pivot it out of engagement with the anvil jaw, the inertia of said rotating hammer member acting to prevent said disengaging motion during an impact blow.

5. A rotary impact tool comprising, in combination, a housing, a motor mounted in said housing, an output shaft mounted on said housing for rotation and including an impact receiving anvil jaw having an impact receiving surface means generally radially disposed on its periphery, a carrier member mounted for coaxial rotation about said output shaft and driven by said motor, a hollow hammer member surrounding said anvil jaw and having an inner surface with impact delivering surface means pivotally mounted in said carrier member for rotation therewith and for angular pivotal motion relative thereto about an axis parallel to but offset from the axis of rotation of said carrier member, said impact delivering surface means located between 0.degree. and 90.degree. clockwise from said pivot connection to the carrier member, said impact delivering surface means and said impact receiving surface means so arranged that upon clockwise rotation of said carrier member said impact delivering surface means contacts said impact receiving surface means to deliver a clockwise impact blow and to produce a counterclockwise pivot torque on said hammer member to tend to pivot said impact delivering surface means out of engagement with said impact receiving surface means.

6. A rotary impact tool comprising, in combination, a housing, a motor mounted in said housing, an output shaft mounted on said housing for rotation about an axis and including an impact receiving anvil jaw generally radially disposed on its periphery, a carrier member coaxially around said output shaft and journaled for rotation in respect to said output shaft, driving connection means between said motor and said carrier member for rotating said carrier member, a hammer member pivotally connected to said carrier member for rotation therewith and for angular pivotal motion relative thereto about an axis offset from but parallel to the said axis of rotation of said carrier member, said hammer member including an impact delivering jaw on its inside surface located within 90.degree. of its connection to said carrier member and so positioned that for operation of the tool in one direction said hammer member pivots relative to the carrier member in that same direction to move said impact delivering jaw into the annular path of rotation of said anvil jaw, cam means to effect said pivot motion, centrifugal force created by the proportions, the mass and the mass center location of said hammer member holding said impact delivering jaw in said pivoted position to deliver an impact blow in that direction to said anvil jaw.

7. A rotary impact tool comprising, in combination, a housing, a motor mounted in said housing, an output shaft mounted on said housing for rotation about an axis and including an impact receiving anvil jaw generally radially disposed on its periphery, a carrier member coaxially around said output shaft and journaled for rotation in respect to said output shaft, driving connection means between said motor and said carrier member for rotating said carrier member, a hammer member pivotally connected to said carrier member for rotation therewith and for angular pivotal motion relative thereto about an axis offset from but parallel to the said axis of rotation of said carrier member, said hammer member including an impact delivering jaw on its inside surface, so positioned that for operation of the tool in one direction said hammer member pivots relative to the carrier member in that said direction to move said impact jaw into the annular path of rotation of said anvil jaw, cam means to effect said pivot motion, centrifugal force created by the proportions, the mass and the mass center location of said hammer member holding said hammer jaw in said pivoted position until it contacts said anvil jaw directly, urging same in said direction and creating an opposite pivot torque on said hammer member, the proportions, mass and mass center location of said hammer member also causing inertia forces during decelleration due to an impact blow sufficient to counter the disengaging pivot motion until the momentum of the carrier and hammer members in said one direction is dissipated.

8. A rotary impact tool comprising, in combination, a housing, a motor mounted in said housing, an output shaft mounted on said housing for rotation about an axis and including an impact receiving anvil jaw generally radially disposed on its periphery, a carrier member coaxially around said output shaft and journaled for rotation in respect to said output shaft, driving connection means between said motor and said carrier member for rotating said carrier member, a hammer member pivotally connected to said carrier member for rotation therewith and for angular pivotal motion relative thereto about an axis offset from but parallel to the said axis of rotation of said carrier member, said hammer member extending from its pivot connection with the carrier member to substantially beyond the said axis of rotation on both sides of the anvil so as to generally surround the anvil, and including an impact delivering jaw on its inside surface so positioned that for operation of the tool in one direction said hammer member pivots relative to the carrier member in that said direction to move said impact jaw into the annular path of rotation of said anvil jaw to contact said anvil jaw directly, urging said anvil jaw in said direction and creating an opposite pivot torque on said hammer member which becomes effective only after the momentum of said rotating members in said direction has been dissipated.

9. The impact tool of claim 8, wherein said hammer member is proportioned, has a mass and a mass center location which cause inertia forces acting on said hammer member while it contacts said anvil jaw during the dissipation of said momentum to overcome said opposite pivot torque.

10. The impact tool of claim 9, wherein said hammer member proportions, mass and mass center location also cause a centrifugal force to hold said impact jaw in the path of said anvil jaw prior to said contact between said jaws.

11. A rotary impact tool comprising, in combination, a housing, a motor mounted in said housing, an output shaft mounted on said housing for rotation about an axis and including an impact receiving anvil jaw generally radially disposed on its periphery, a carrier member coaxially around said output shaft and journaled for rotation in respect to said output shaft, driving connection means between said motor and said carrier member for rotating said carrier member, a hammer member pivotally connected to said carrier member for rotation therewith and for angular pivotal motion relative thereto about an axis offset from but parallel to the said axis of rotation of said carrier member, said hammer member including an impact delivering jaw on its inside surface capable of being pivoted into and out of the path of the anvil jaw, cam means to pivot said impact delivering jaw into said path, centrifugal force holding said impact delivering jaw in said path until impact with the anvil jaw, and cam means to pivot said impact delivering jaw out of said path including impact surfaces of said hammer and anvil jaw being shaped so that during the impact of said hammer member with said anvil jaw a camming force is created acting on the hammer member tending to cause the hammer jaw to pivot outwardly to disengage from said anvil jaw, and said hammer member being so positioned, proportioned and having a mass and a mass center location which will cause the creation of inertia forces acting on said hammer member during the interval of said impact until the momentum of said hammer and carrier members is dissipated, thereby overcoming said camming force and preventing said hammer jaw from pivoting outwardly during the interval of impact, said mass center being closer to the carrier axis than to the hammer pivot axis.

12. A rotary impact tool comprising, in combination, a housing, a motor mounted in said housing, an output shaft mounted on said housing for rotation about an axis and including an impact receiving jaw generally radially disposed on its periphery, a carrier member coaxially around said output shaft and journaled for rotation in respect to said output shaft, driving connection means between said motor and said carrier member for rotating said carrier member in a given direction, a hammer member pivotally connected to said carrier member for rotation therewith and for angular pivotal motion relative thereto about an axis offset from but parallel to the said axis of rotation of said carrier member, said hammer member including an impact delivering jaw on its inside surface, said hammer member being proportioned, and having a mass and a mass center location which will cause the creation of a centrifugal torque acting on said hammer member prior to the impact of said jaws to hold said hammer jaw pivoted inwardly in position to strike said anvil jaw prior to said impact and will cause the creation of inertia forces acting on said hammer during rebound of the hammer following said impact, after the momentum of said rotating members in said given direction has been dissipated, that will cause said hammer jaw to pivot outwardly to a position where it can rotate past said anvil jaw during further rotation in said given direction.

13. A rotary impact tool comprising, in combination, a housing, a motor mounted in said housing, an output shaft mounted on said housing for rotation about an axis and including an impact receiving anvil jaw generally radially disposed on its periphery, a carrier member coaxially around said output shaft and journaled for rotation in respect to said output shaft, driving connection means between said motor and said carrier member for rotating said carrier member, a hammer member pivotally connected to said carrier member for rotation therewith and for angular pivotal motion relative thereto about an axis offset from but parallel to the said axis of rotation of said carrier member, said hammer member including an impact delivering jaw on its inside surface, cam means to move said impact delivering jaw inwardly into the path of rotation of said anvil jaw, said hammer impact jaw being located relative to the hammer pivot axis so that the impact between said hammer impact delivering jaw and said anvil jaw will create compressive stresses in said hammer member between its impact delivering jaw and said pivot axis, said hammer member being proportioned and having a mass and a mass center location which will cause the creation of a centrifugal torque acting on said hammer member prior to the impact between said impact jaws to hold said hammer jaw pivoted inwardly in position to strike said impact.

14. The wrench mechanism of claim 13 wherein said hammer member extends around said anvil over an angle of at least 170.degree..

15. The wrench mechanism of claim 13 wherein said hammer member extends completely around said anvil.

16. The wrench mechanism of claim 13 wherein the mass center of said hammer member is closer to the axis of said carrier member than to said pivot axis of said hammer member.

17. The wrench mechanism of claim 13 wherein the impact surfaces of said hammer jaw and anvil jaw are shaped so that during the impact of said hammer jaw with said anvil jaw a camming force is created acting on the hammer member tending to cause its jaw to pivot outwardly to disengage said jaw during said impact, and said hammer member being also proportioned and having a mass and a mass center location which will cause the creation of inertia forces acting on said hammer member during the interval of said impact that will overcome said camming force and will prevent said hammer member from pivoting outwardly during the interval of impact.

18. The wrench mechanism of claim 13 wherein said hammer is positioned, proportioned and having a mass and a mass center location that will cause the creation of inertia forces acting on said hammer during rebound of the hammer following the impact of said impact surfaces that will cause said hammer to tilt outward to a position where it can rotate past said anvil jaw during further forward rotation.

19. The wrench mechanism of claim 13 wherein a pair of said anvil jaws are mounted on said output shaft and angularly and longitudinally spaced from each other; and a pair of said hammer members is pivoted in said carrier member in an angular and longitudinal spaced relationship and arranged to impact simultaneously with said anvil jaws.

20. The wrench mechanism of claim 19 wherein said anvil jaws are angularly spaced 180.degree. from each other.

21. A rotary impact tool comprising, in combination, a housing, a motor mounted in said housing, an output shaft mounted on said housing for rotation about an axis and including an impact receiving anvil jaw generally radially disposed on its periphery, a carrier member coaxially around said output shaft and journaled for rotation in respect to said output shaft, driving connection means between said motor and said carrier member for rotation said carrier member as said motor is rotating, a hammer member pivotally connected to said carrier member for rotation therewith and for angular pivotal motion relative thereto about an axis offset from but parallel to the said axis of rotation of said carrier member, said hammer member including an impact delivering jaw on its inside surface located within 90.degree. of its pivot connection on said carrier member and so positioned that for operation of the tool in one direction said hammer member pivots relative to the carrier member in that same direction to move said impact delivering jaw into the annular path of rotation of said anvil jaw to deliver an impact blow in that direction, and cam means on said hammer member located and adapted to engage said anvil jaw to pivot said impact delivering jaw into the path of said anvil jaw at least 90.degree. prior to the delivery of said impact blow, said hammer and anvil jaw allowing said impact delivering jaw to remain in the path of said anvil jaw while said hammer member rotates through said 90.degree. angle ending at the delivery of said impact blow.

22. The rotary impact tool of claim 21 wherein said hammer member extends at least 170.degree. around said output shaft.

23. The rotary impact tool of claim 21 wherein said cam means on said hammer member is disengaged from said anvil jaw for at least the last 90.degree. prior to striking the impact blow.