Oriσin of the Invention

This invention was made by an employee of the United States Government and may be used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. Field of the Invention

This invention relates generally to mechanical brake and clutch devices and more particularly to roller type locking sprag devices which operate to couple torque between a drive member and a reaction or fixture member.

Description of the Prior Art

Locking techniques using ball or roller members are well known and have been used extensively in various types of clutch mechanisms. In such apparatus, the motion of one member, referred to as a driver member, forces a ball or roller to wedge against an inclined surface of another member, referred to as a reaction or fixture member. A sprag, to which the present invention is directed, comprises a roller having a line contact bearing surface as opposed to a point contact surface of a ball and is used where relatively large loads and contact stresses are encountered. Sprags typically have mutually opposing or slightly offset two dimensional cylindrical contact surfaces located between a drive

surfaces located between a drive member and a reaction member and having a body which comprises a portion of a roller and which are spring biased against the contact surface of the drive member and the reaction member. 5 Known prior art type sprags typically have relatively small cam angles or inclines, normally on the £ order of 6°. Thus, they must have relatively large contact stresses in order to generate sufficient holding torque when transferring torque from the drive member to

10 the reaction member. The combination of these large contact stresses and the small cam angles requires large radial forces which in turn, force the walls of the device to be thick and long. It also forces the sprags to be relatively long. This results in sprag devices

15 utilized, for example, in over-running clutches, to be heavier and bulkier than necessary. Also, the machining tolerances on the cams must be very close because if a 6° cam is machined too closely, the roller tries to move over center and jams to the point where it will not come

20 out and excessive forces in clutch damage may result. If, on the other hand, the cam angle of the incline surface is too large, such as being greater than 6°, the clutch will slip. For example, in space where the coefficient of friction can vary by as much as 25%, a 6°

25. cam can be vulnerable in its utilization.

Summary

It is an object of the present invention, therefore, to provide an improvement in roller type locking brakes and clutches. 5 It is another object of the invention to provide an improvement in roller-type locking sprag systems. i It is a further object of the invention to provide a sprag system which is more effective and reliable in its ability to provide a lock-up between a drive member 10 and a reaction member.

It is yet another object of the invention to provide a sprag member which is relatively shorter in axial length than conventional sprags without any sacrifice in performance. 15 It is yet a further object of the invention to overcome the requirement of relatively thick and long sprag members resulting from large contact stress and small cam angles.

It is still a further object of the invention to

20 provide a three-dimensional sprag system having at least one sprag member which is confined to a groove so that it cannot migrate axially between a drive member and a reaction member.

Briefly, the foregoing objects and advantages are 25. realized by a torque coupling sprag system which is

three-dimensional in nature where the change is effected from a cylindrical type sprag contact to a contact between the sides of at least one groove and at least two contact surfaces angled with respect to the central axial 5 direction of at least one sprag member located between a drive member and a reaction member. The sprag member is . rotatable between the reaction member and the drive member, which have an angle therebetween defined by a straight line passing through a central rotatable axis

10 and a first pair of contact surfaces on the drive member and a straight line passing through the central axis and a second pair of contact surfaces on the reaction member. The surface contacts between the members include various combinations of diametrically opposed flat and curved

15 contact surface segments arranged in a predetermined manner to define a desired configuration. Brief Description of the Drawings

The following detailed description of the invention will be more readily understood when considered together

20 with the accompanying drawings wherein:

Figure 1A is a side elevational view of a typical known prior art sprag device;

Figure IB is a cross sectional view of the embodiment shown in Figure 1A taken along the lines IB-IB

25. thereof;

Figures 2 and 3 comprise side elevational views illustrative of other prior art sprag devices;

Figure 4A is a side elevational view of a first embodiment of the subject invention; 5 Figure 4B is a cross sectional view illustrative of the embodiment shown in Figure 4A taken along the lines 4B-4B thereof;

Figures 5 and 6 are side elevational views of second and third embodiments of the subject invention; 10 Figures 7A-7D are partial cross-sectional views illustrative of a first set of modified versions of the embodiment shown in Figures 4A and 4B including male type sprags and female type fixtures;

Figures 8A-8D are cross-sectional views of a second- 15 set of modified versions of the embodiment shown in Figures 4A and 4B including female type sprags and male type fixtures. Detailed Description of the Invention

Referring now to the figures wherein like reference 20 numerals refer to like parts throughout, reference will first be made to Figures 1A and IB which are illustrative of a conventional two-dimensional (2-D) roller locking sprag arrangement comprising a cylindrical type sprag member 10 located between a rotary type drive member 12 25. and a rotary type reaction member 14, which may also be

referred to as a fixture. The reaction member or fixture 14 has an inclined roller contact or cam surface 15 which is inclined at an angle θ with respect to a Y axis of an orthogonal X, Y, Z axis system passing through the center 5 of the sprag member 10 and which makes contact therewith at contact point 16. The sprag member 10 also makes contact with the driver member 12 at contact point 20.

As depicted in Figure IB, the sprag member ^{^} 10 has a thickness dimension along the Z axis which results in

10 contact with the driver and reaction members 12 and 14 along the lines 22 and 24. The contact point 16 shown in Figure 1A lies along the line 22 and the contact point 20 on the line 24. Further, the sprag member 10 is spring biased against the drive member 12 and the reaction

15 member 14 by a preload spring member consisting of a compression spring 26 causing tangential frictional forces to build up along the contact surface lines 22 and 24 between the surfaces 11, 13 and 15.

In operation, torque is coupied from the drive

20 member 12 to the reaction member 14 when, as shown in Figure 1A, the drive member 12 rotates in a clockwise direction. This causes the sprag member 10 to rotate around the Z axis in a counterclockwise direction and roll up the inclined surface 15 of the reaction member

25. 14. The sprag member 10 will wedge itself between the

reaction member 14 and the drive member 12 with frictional forces Fp-j_ and F 2 developing, at which time lock-up occurs and the torque provided by the drive member 12 will be transferred or coupled to the reaction member 14 which will then also rotate in a clockwise direction along with the drive member 12. v' At lock-up, in addition to the frictional forces Fp^ and F 2, there exists a force F which is applied by the reaction member 14 to the sprag member 10 directed toward the Z axis and normal to the inclined surface 15 as well as a reaction force directed from the driver member 10 directed along the Y axis. Further at such time the summation of the forces along the X axis and Y axis and the moments about the Z axis are all equal to zero. Referring to Figures 1A and IB, these relationships can be expressed as follows. For the summation of forces along the X axis:

ΣF _X « 0 (1) and it can be shown that F sin θ - Fpi cos θ + Fp2 (2)

With respect to the forces along the Y axis,

ΣF _y - 0, (3) and

F cos θ + F i sin θ ^■ = FR (4) - With respect to the moments about the Z axis, they are

also equal to zero, such that, r.M _z = 0, (5) therefore

F _F1 = F _{F2 (} 6 ₎

5 Typically sprag clutches are designed such that Fp2 slips first during unlocking of the elements 10, 12 and 14, then

*F2 - ^F R s ⁽ 7 ⁾ where μ _s is a coefficient of friction between the sprag 10 member 10 and the driver member 12.

Using equations (2), (6) and (7), it can be further shown that

F sin θ « Fp2 (1 + cos θ) - FR μ _s (1 + cos θ) (8) . Accordingly, 15 F sin Θ/F _R (1 + cos θ) ≤ μ _a (9)

Depending upon the value of μ _s which is a limit indicative as to when slippage occurs, the maximum angle of θ is governed by the relationship as expressed in equation (9) and for 2-D sprags (Figures 1A and IB) . The 20 sprags are normally operated in a lubricant. Accordingly, μ _s is small resulting in a small θ, typically on the order of 6°.

Prior to considering the embodiments of the subject invention, reference will first be made to Figures 2 and

25. 3 which are illustrative of 2-D sprag systems which

utilize only a partial section of a cylindrical roller. As shown in Figure 2, the sprag member 10' comprises a body member having a pair of curved surfaces llτ. and H2 which respectively contact the inclined surfaces 15 and 17 of the reaction member 14' and the drive member 12' at contact points 16 and 20, respectively. In this embodiment, both inclined surfaces 15 and 17 have equal cam angles (θ) . It should be noted, however, that these cam angles do not have to be equal. They are shown as such for the sake of simplicity and thus are not meant to be limited thereto.

Now instead of having a single preload bias spring 26 as shown in Figure 1A, the sprag member 10' of Figure 2 includes a second preload spring 27 so that a spring bias is applied above and below the center or pivot point 18. The configuration of Figure 2 is one in where a linear motion of the sprag member 12' is transferred or coupled to the reaction member 14' . This is due to the fact that when the sprag member 12' moves to the right as shown, the surface II2 of the sprag member 10' rolls up the incline surface 17 and in doing so, the upper curved surface ll-t_ rolls up the incline surface 15 of the reaction member 14', causing it to lock-up in a conventional manner. A movement of the drive member 12' • in the opposite direction releases the coupling between

the drive member 12' and the reaction member 14' as the sprag member 10' .

Referring now briefly to Figure 3, it includes a sprag member 10'' which is structurally similar to that shown in Figure 2, with the exception that rather than the contact points 16 and 20 being aligned with the pivot 18, in the configuration shown in Figure 2, offset radii are contemplated and as a result, the contact points 16 and 20 are mutually offset from one another on either side of the pivot 18. Also, the contact surfaces between the sprag member 10'' and the drive member 12'' and the reaction member 14'' are curved surfaces as shown by reference numerals 13' and 15' .

Directing attention now to the preferred embodiments of the subject invention, Figures 4A and 4B are illustrative of a torque coupling system where the driver member 32 and the incline, i.e. the reaction member 34 are both grooved and the sprag member 30 rolls along a path common to both grooves, with two side surfaces of the sprag member making contact with diametrically opposing sides of the grooves.

As illustrated in Figure 4B, the drive member 32 includes a groove 36 having a pair of mutually diverging side wall surfaces 38 and 40 which contact a pair of curved peripheral side surfaces 42 and 44 of the sprag

member 30. The pair of side wall surfaces 42 and 44 of the sprag member 30 contact the side wall surfaces 38 and 40 of the driver member 32 in the diametrically opposing regions 46 and 48 and which are located at a radius R*-_ from the rotational or Z axis 50 which forms one of the three mutually orthogonal axes X, Y and Z in Figures 4A and 4B. The curved peripheral side surfaces 42 and 44 of the sprag member 30 also contact a pair of mutually diverging side wall surfaces 52 and 54 of a groove 55 formed in the body of the reaction member 34. The peripheral contact surfaces 42 and 44 contact the side walls 52 and 54 in the diametrically opposing regions 56 and 58 which are also located at the radius R^ of the drive member 32. Further, as shown in Figure 4B, both pairs of side wall surfaces 38, 40 and 52, 54 of the driver member and reaction member, respectively, are of the same type, i.e. flat surfaces which respectively contact curved, i.e. convex peripheral side wall surfaces 42 and 44 of the sprag member 30. It should be noted that the radius of curvature R2 of the surfaces 42 and 44 is extremely large relative to R**_. This permits the sprag member 30 to roll in the grooves 36 and 55 of the drive and the reaction members 32 and 34 via pairs of mutually opposing contact surfaces.

What is provided is a three-dimensional (3-D) sprag device but which now involves a second angle φ as shown in Figure 4B. The angle φ is the angle between a line between the points of contact 46, 48 and 56, 58 and radius of curvature R2 and defines both the angle of the grooves 36 and 55, and the angle of the contact surfaces 42 and 44 of the sprag member 30.

In such a configuration, the reaction force F*-* _* now includes a term sin φ and which can be expressed as F'- _j -. and where,

^F 'R - F sin φ (10) in which case, equation (9) is now expressed as,

(F sin θ) (sin φ)/F***t(l + cos θ) ≤ μ' _s / _1: - where μ' _s is the coefficient of friction for the 3-D embodiment (Figures 4A and 4B) and, therefore 's - Ms ^sin - ⁽ 12 ⁾

This indicates that the angle θ for a 3-D embodiment can be inherently larger than the angle θ for a 2-D configuration as shown in Figures 1A and IB. This also means, that the locking capabilities of a 3-D roller type sprag member such as shown in Figure 4B is improved by a factor of 1/sinφ. For a contact angle of θ . 30°, for example, this effectively doubles the locking effectiveness. This locking effectiveness can be utilized in, for example, three different ways. If the

angle θ of the incline and the coefficient of static friction (μ' _s ) remain the same, the margin of safety and reliability of locking over slipping increases several times depending upon the sprag-groove contact angle. On 5 the other hand, one can obtain the same margin of safety and reliability and using different materials and more slippery lubricants without penalty. That is, the static coefficient of friction can be reduced or in some ways relaxed. The third way of utilizing and improving 10 locking effectiveness is to increase the angle θ of the incline while keeping the static coefficient of friction μs the same. This has the effect of increasing the torque output of a brake/clutch device employing locking sprags without increasing the contact stresses on the 15 sprags.

At the same time, very compact 3-D sprags can withstand very large forces and still maintain modest contact forces. Contact stresses for a point contact between 2 spheres on which one sphere is inside the other

20 are calculated as shown in equation (13) below:

P ₀ - (1/Rτ. - 1/R ₂ ) (E/2)2p]l/3 (13) where:

P ₀ is the contact stresses, Rτ_ is the inside radius (sprag) , 25. R2 is the outside radius (groove) ,

E is the materials modulus of elasticity and P is contact load.

With 3-D sprags, the contact radius R2 as shown in

Figure 4B acts against a flat surface, for example 52 and 54, as shown; however, R2 is independent of the sprag roll radius i in Figure 4A. Thus R2 *> Ri without effecting the size of the sprag 30. R2 in Figure 4B corresponds to R-^ in the equation (13) and "the flat grooves in Figure 4B correspond to P2 in the equation where R2 → ∞ -

Thus with a very large contact radius operating against an opposing e.g. flat surface, very large forces can be withstood with minimal contact stresses. Studies have shown that if one uses, P ₀ - 2K1/R! - 1/R ₂ ) (E/2)2p]l/3 (14) a conservative approximation is obtained (within 10%) in the 3-D sprag case.

It is clear that P ₀ can be kept very small despite large P loads and this entirely independent of the sprag roll radius R**..

Furthermore, a 3-D sprag geometry has significant advantages in the performance of both the inner disk or driver member 32 and the outer ring or reaction member 34 in Figures 4A and 4B respectively. The 2-D case of Figure 1A and Figure IB shows a convex surface of a 2-D

sprag 10 bearing on a convex surface of an inner disk 13. This arrangement is inherently prone to large stress concentrations and is somewhat analogous to P ₀ in equation (13) . Therefore, it is not surprising that, 5 classically, the inner disk/sprag surface is the weak point of sprag clutch/brake devices.

With 3-D sprag techniques, however, this problem is by passed. The dominant effect is a convex sprag surface bearing, for example, against a flat or curved reaction

10 or driver surface, with the radius R2 of the sprag surface as large as desired, thus keeping the contact stresses low as permitted in the previously shown equation (14) .

The 3-D sprag geometry also presents advantages for

15 the outer ring or reaction member. In the 2-D geometry, the reaction member 14 (Figure 1A) , 14' (Figure 2), and 14'' (Figure 3) must be made both long in axial length and thick to guard against bending deformations brought about by the very large bearing forces. Elastic (and

20 plastic) bending deformations are caused, in large part, by the geometric moment of inertia of the cross section of this member. In the case of the 2-D sprag, this is a long, narrow rectangle which has as its rotational center, a line through the center of the rectangle in the

25. axial direction. This geometry is very weak in bending.

Increasing the axial length, a linear function does little to help; thickness, a cubic function, is required. In the 3-D sprag case, the flange thicknesses of the reaction member 34 (Figure 4B) which comprise the contact surfaces with the sprag member 30 provide this thickness and thus this geometry is very resistant to bending deformations. This results despite it being relatively short in axial length and despite very thin walls in the region between the outer limit of the groove 56 and the outside of the reaction member 34. Also, 3-D geometry permits thick flanges without requiring a corresponding increase in the overall diameter of the device.

Further, all that is required to convert a 2-D sprag device to a 3-D device is to change from cylindrical contact as shown in Figure IB to a contact between diametrically opposing sides of the grooves and four contact surfaces angled with respect to the axial direction of the sprags as shown in Figure 4B. Thus the 2-D configurations of Figures 2 and 3 ^* can be converted to 3-D configurations as shown in Figures 5 and 6 by the inclusion of grooves 36 and 55 in the drive member 32' and reaction member 34' of the configuration of Figure 5 and grooves 36 and 55 in the drive member 32'' and the reaction member 34' ' in the configuration shown in Figure 6. In both instances, the sprag members 30' and 30'' now

include a pair of spaced peripheral contact surfaces having a radius R-*_ as shown in Figure 4B and consisting of the surfaces 42 and 44.

It is not essential that a 3-D sprag system be comprised of a combination of flat and rounded contact surfaces as configured in Figure 4B, nor is it necessary that the sprag member 30 be a male-type member while the drive members 32 and the reaction member 34 be female type members. Figures 7A - 7D, for example, disclose four variations of surface contact configurations of Figure 4B for a male sprag and a female type reaction member. In Figure 7A, the sprag member 30 _a includes a pair of convex contact surfaces 42 and 44 such as shown in Figure 4B; however, the reaction member 34 _a , includes a pair of convex contact surfaces 60 and 62 in the groove 55a instead of the flat contact surfaces 52 and 54. As to the arrangement shown in Figure 7B, the sprag member 30-^ includes a pair of convex contact surfaces 42 and 44 like that shown in Figure 7A; however, the reaction member 34b now includes a pair of concave contact surfaces 64 and 66 in the groove 55]-,. With respect to the scheme shown in Figure 7C, the sprag member 30 _c includes a pair of flat contact surfaces 68 and 70, while the reaction member 34 _c includes a pair of convex contact surfaces 60 and 62 such

as shown in Figure 7A. In Figure 7D, the configuration is shown where the sprag member 30^ includes a pair of concave contact surfaces 72 and 74, while the reaction member 34^ includes a pair of convex contact surfaces 60 5 and 62 in the groove 55^.

Turning attention now to Figures 8A-8D, female type sprag members 30 _e ... 3Oft engage male type reaction members 34 _€ ... 34^- In Figure 8A, there is" shown a female type sprag member 30 _e including a pair of convex

10 surfaces 76 and 78 in a groove 80 _e , while the male type reaction member 34 _e includes a pair of convex contact surfaces 82 and 84. In the embodiment of Figure 8B, the reaction member 30f also includes a pair of convex contact surfaces 76 and 78, while the sprag member 34f

15 includes a pair of flat contact surfaces 86 and 88. As to the embodiment of Figure 8C, it discloses a reaction member 30g having a pair of concave contact surfaces 90 and 92 which abut a pair of convex contact surfaces 82 and 84 on the sprag member 34g. When desirable, both

20 sets of contact surfaces 90, 92 and 82, 84 may comprise continuous curved surfaces. Finally, with respect to the embodiment shown in Figure 8D, it depicts a reaction member 30*^ having a pair of convex contact surfaces 76 and 78 in a reaction member groove 80*^ and which are

25. adapted to contact a pair of concave contact surfaces 94

and 96 in the sprag member 34*^.

It should be noted that it is possible to have any number of variations of the schemes shown in Figures 7A- 7D and 8A-8D. For example, one can have the top of the sprag as a groove, and the bottom as a male member which will mate with corresponding ridges and grooves in the reaction and drive members, respectively, or vice versa. Also, the contact angles of the top and bottom of the sprags can be different, depending upon the performance desired.

Having thus shown and described what is at present considered to be the preferred embodiments of the invention, it should be noted that the same has been made by way of illustration and not limitation. Accordingly, all alterations, modifications and changes coming within the spirit and scope of the invention are herein meant to be included.