GB2054798A - Fluid pressure actuators - Google Patents

Abstract

A fluid pressure actuator assembly comprises a pair of coaxial pistons 10 between which is an elongate body 11 having a toothed rack 12 on one side, for engaging a pinion 21. Each piston 10 is rigid with a stem 13 slidable with limited lateral freedom in the body 11. The body is guided at spaced locations therealong by support means 19, 19. Assemblies involving a single piston only are also described. <IMAGE>

Description

SPECIFICATION Improvements in fluid pressure actuators A common type of semi-rotary fluid pressure actuator includes two opposed pistons joined by a rack portion, which meshes with a pinion fixed to a shaft, so that reciprocation of the pair of pistons causes to-and-fro rotation of the shaft. Such actuators are, for example, used as air pressure operated windscreen wiper motors.

One common construction is to have the rack integral with the pistons, resuiting in a dumb-bellshaped member. The inclination of the flanks of the rack teeth means that the force between the rack and the pinion has a component transverse to the length of the rack. Reaction to this transverse component is provided by the engagement between the pistons and the walls of the cylinders in which the pistons operate. This has the disadvantage that the side load is imposed on the piston seals, and this is detrimental to the life of the seals. A further disadvantage is that it may be difficult to control the distance between the rack pitch plane and the axis of the pinion shaft, which controls the backlash of the gearing.If a separate support is provided at the centre of the dumb-bell to take the lateral load, and to control the backlash, (with suitable adjustment provision if necessary), the result is that the composite sliding member comprising the rack and the pair of pistons is supported at more than two points, with consequent alignment problems.

Another type of semi-rotary fluid pressure actuator includes a single piston secured to a rack extending from one face of the piston. In one such construction. the rack is supported in engagement with the pinion by an adjustable roller. lo use, the transverse component of force between the rack and the pinion causes side load on the piston.

It is also known for two pistons to be portions of a dumb-bell shaped member, while the rack is a separate member, with its own support, between the pistons. This can eliminate the above problems, but still leaves the piston pair as a dumb-bell shaped member. If this member is to be made under mass production conditions, it is desirable, for the sake of cheapness, for it to be a metal casting or a plastics moulding. With a simple mould, the mould split line runs across the diameter of the piston lands and across the groove in each piston which carries a seal, leaving a mark which requires to be removed by machining, in order to avoid leakage past the seal. It is possible to avoid this only by using more complicated and expensive moulds, with side cores.

The object of the present invention is to provide constructions for a semi-rotary actuator which are such that the rack can be supported independently of the interengagement of the piston or pistons and cylinder or cylinders, and the piston or pistons can be made as metal castings or plastics mouldings, in a simple mould, without requiring subsequent machining.

An actuator assembly according to the present invention comprises a piston and an elongate body extending away from the piston, the body having a toothed rack along one side for meshing with a pinion and moving with the piston and having an abutment surface at each end for contact and driving by the piston during movement of the piston in one sense and by means for returning the body during movement of the piston in the opposite sense, respectively, and support means for the body to maintain the rack in mesh with the pinion, characterised in that the piston has a stem rigid therewith and supported with limited lateral freedom, and the support means act directly on the body at spaced locations therealong to ensure that reciprocation of the body is rectilinear when the rack lies in correct relationship to the pinion.

Preferably, for economy, each piston and stem is identical, if there are two.

In one particular construction, each stem is a rod of semi-circular cross section (i.e. one half of a cylinder), and the two stems when assembled side by side are a loose fit in a cylindrical bore in the body, and are thus located by the body.

In another particular construction, the body is located relatively to the pinion by guide means in a housing, and each stem is guided in a bore through this housing.

The accompanying drawings show examples of actuators according to the present invention. In these drawings Figure 1 is a longitudinal section of one actuator; Figure 2 is a section on the line Il-Il in Figure 1 with the rack removed; Figures 3 and 4 are cross sections, showing alternative constructions; Figure 5 is a iongitudinal section of a second actuator; Figure 6 is a side elevation, looking in the direction of the arrow VI in Figure 7; Figure 7 is a plan, and partial section on the line VIl-VIl in Figure 6; Figure 8 is a section on the line VIll-VIll in Figure 5; Figure 9 is an enlargement of the central part of Figure 8, with width exaggerated; Figure 10 is a longitudinal section of a third actuator;; Figure 11 is a fragmentary section on the line Xl-XI in Figure 10; Figure 12 is a longitudinal section of a fourth actuator; and Figure 13 is a section on the line XIll-XlII in Figure 12.

In the actuator shown in Figure 1 thp h.iy 1,1 is a bar of rectangular cross section, with a toothed rack 12 along one side. Through the centre of the body there is a cylindrical bore 23, with its axis parallel to the pitch plane 1 2a of the rack 12.

There are two identical pistons 10, each integral with a stem 13. Each stem 13 is a semicylindrical rod, slightly shorter than the body 11, and the two stems 1 3 are a loose fit in the bore 23 in the body 11 (on the scale used, the looseness is not visible in the drawing).

A separate disc 1 5 is fitted against the outer end of each piston 10, and is held captive by three pegs 1 6 integral with the piston, the ends of which are expanded under heat and pressure. The disc 15, in conjunction with a step in the piston 10, defines a groove which receives an O-ring seal 17. The piston 10 with its integral stem 13 is made as a plastics moulding or metal casting in a two-part mould which separates in the plane A-A, or in the plane B-B. In consequence, the O-ring 1 7 cooperates with annular surfaces which are not marked by any lines of separation of mould parts.

In normai operation, the pistons abut against end surfaces 14 of the body 1 , these surfaces 14 being perpendicular to the axis of the body.

In use, the assembly of body 11 and piston 10 is inserted into a housing 9, having a cover 20 in which is journalled a shaft 32 carrying a pinion 21 which meshes with the rack 12. The housing has two end portions 22 which constitute cylinders accommodating the pistons 10. Either the cylinders are in the form of pots bolted to the central housing 9 (as shown), or the cylinders have removable end covers but are themselves integral with the central housing. In normal operation, the pressure in the central housing is atmospheric, whereas the pressure in the cylinders, acting on the outer faces of the pistons, is alternately substantially above atmospheric, and slightly above atmospheric (because of the presence of some restriction in the exhaust). Thus the pistons are held against the end surfaces 14 of the body.

However, the stems 13 are of sufficient length to ensure that, even if the pistons 10 are simultaneously at the outer ends of the two cylinders, the stems 1 3 do not disengage from the body 11, and the components can return without difficulty to the relative condition shown in Figure 1.

In the side of the body 11 opposite the rack 12 there is a groove 18, and this can cooperate with the ends of the abutment screws 1 9, which are mounted in the housing 9. These screws can be adjusted so that the pitch plane 1 2a of the rack 12 lies in the correct relationship to the pinion 21, and the combined clearance or lateral freedom of the stems 13 in the bore 23 in the body 11 is sufficient (in practice up to 1/64 inch (0.38mm) is tolerable) to enable this adjustment to take place without imposing any side load between the pistons 10 and the walls of the cylinders 22.

Indeed the clearance may be sufficient to permit simplified adjustment of the pitch plane by moving one only of the screws 1 9. This may result in the rack axis being slightly tilted in relation to the axis of the cylinders, but a slight tilt is harmless. After adjustment, the screw is secured by a sealant.

Alternatively, the relationship of the pinion to the rack may be adjusted by shims at 35 under flanges 36 on the cover, and at the bottom of a corresponding recess in the housing 9.

As screws 1 9 act directly on body 11 at spaced locations therealong and the outer diameter of the screws corresponds to the width of groove 18, reciprocation of body 11 is rectilinear when the screws 1 9 or shims 35 have been adjusted so that the rack lies in correct relationship to pinion 21.

Figure 3 shows an alternative construction in which the semi-cylindrical stems 13 are replaced by two concentric stems 1 3a and 1 3b.

Figure 4 shows another alternative construction in which the body 1 c is a rectangular-sectioned box such as could be made by die casting or plastics moulding, and the two stems 1 3c and 1 3d are flat rectangular-sectioned bars.

In the actuator shown in Figures 5 to 9, the body 11 d is built up of a number of identical laminations 25 of steel strip. Each of these laminations is formed with rack teeth 1 2d before the laminations are assembled. The laminations are located accurately in relation to one another by half bursts 26 (i.e. material partially displaced, to form a projection on one face of a lamination, and a corresponding recess in the other face). The laminations are held together by transverse rivets.

Because of the hardness of the steel strip, the total width of the body 1 ldcan be less than that needed if the body is cast.

There are two identical pistons 1 Od, each integral with a stem 13d. Each stem 13dips a round rod, offset from the central axis of the pistons 1 Od, as can be seen in Figure 8.

In this construction, the central portion 9dof the housing is mainly solid, with a cavity to accommodate the pinion 21 d, which meshes with the rack 12d. The rack 12dips guided for rectilinear reciprocation in a slot 27 in the housing portion 9d, while each of the stems 1 3d is guided in a respective bore 28 in the housing portion 9d, each stem having limited lateral freedom in its respective bore. The pinion 21 d is fixed to a shaft 32d, which is journalled at 33 in the housing portion 9d and at 34 in a plug 35 which closes one side of the cavity for the pinion 21 d.

In this arrangement, as can be seen in Figure 5, the end portions of the body 11 dare cranked, so that the pitch plane of the rack 1 2d coincides with the common axis of the pistons 1 Od, while at the same time each end face 1 4d of the body 11 d cooperates with the centre of one face of a respective piston 1 Od.

The process of manufacturing the laminations 25 tends to produce a slightly irregular lower surface of the body 11 d. In order to provide smoother sliding guidance in the slot 27., the lower part of the body is enclosed in a cover 29 of channel-shaped cross section. In order to adjust the backlash between the.rack 12dand the pinion 21 d, one or more shims 30 may be inserted between the laminations 25 and the web of the cover 29. As shown in Figure 9, there is a notch in the right-hand flange of the cover 29 to clear the adjacent half burst 26.

In the actuator shown in Figures 10 and 11, there is a single piston 41 integral with a cylindrical stem 42. A groove containing a seal 43 is defined by a disc 44 secured to the piston.

The stem 42 is a loose fit in a cylindrical bore in a body 45. This body carries rack teeth 46 meshing with a pinion 47. On the face of the body opposite the rack teeth, there is a groove 48, which cooperates with the tips of two guiding screws 49, 50.

A collar 51 is secured to the right-hand end of the stem 42. There is a small amount of longitudinal play, because the distance between the collar 51 and the piston 41 isslightly.greater than the length of the body 45. This play, and the looseness of the stem 42 in the bore in the body, are not sufficient to be visible in the drawings, but they ensure that the vertical component of the force between the pinion 47 and the rack teeth 46 is transmitted to the screws 49, 50, and not to the piston 41.

In the actuator shown in Figures 12 and 13, the piston 52 has secured to it two parallel cylindrical stems 53, the right-hand ends of which are linked together by a tie member 54 (see particularly Figure 13). Between the piston 52 and the tie member 54 there lies a body 55, made of laminations, as described earilier. The body has rack teeth 56, and the ends 57, 58 are cranked, so that the pitch plane of the rack coincides with the axis of the piston 52 and the centre of the tie member 54. There is a small amount of longitudinal play between the ends of the body and the piston and the tie member.

The stems 53 are guided loosely in bores in a central housing portion 59. The body 55, which includes a surrounding channei-shaped portion of self-lubricating material 60, is separately guided in a slot 61 in the housing portion 59. The rack 56 meshes with a pinion 62 on a shaft 63.

Claims (12)

1. A fluid pressure actuator assembly comprising a piston and an elongate body extending away from the piston, the body having a toothed rack along one side for meshing with a pinion and moving with the piston and having an abutment surface at each end for contact and driving by the piston during movement of the piston in one sense and by means for returning the body-during movement of the piston in the opposite sense, respectively, and support means for the body to maintain the rack in mesh with the pinion, the piston having a stem rigid therewith and supported with limited lateral freedom and the support means acting directly on the body at spaced locations therealong to ensure that reciprocation of the body is rectilinear when the rack lies in correct relationship to the pinion.

2. An assembly according to claim 1 and comprising a.pair of pistons arranged on a common axis at opposite ends of the body, one of the pistons being the said piston and the other constituting the said means for returning the body, said other piston also having a stem rigid therewith and supported with limited lateral freedom.

3. An assembly according to claim 2, in which the stems of the pair of pistons extend in a common passage through the body.

4. An assembly according to claim 2, in which the portions of the body at opposite ends thereof are cranked so that the pitch plane of the rack coincides with the common axis of the pistons, and the body is arranged to contact the centre of each piston.

5. An assembly according to any of claims 2 to 4, in which each piston and stem combination is identical.

6. An assembly according to claim 1, in which the piston stem is contained in a longitudinal passage in the body.

7. An assembly according to claim 1, in which the body is guided for reciprocation in a housing which also guides the piston stem individually for sliding.

8. An assembly according to claim 1 or 7, in which the piston stem extends in the pitch plane of the rack.

9. An assembly according to claim 7, in which the piston stem is one of a pair equispaced from the piston centre and separated by 1 80O to extend on opposite sides of the body and be guided for sliding in the housing, the means for returning the body comprises a tie member secured on the pair of piston stems, portions of the body at opposite ends thereof are cranked so that the pitch plane of the rack coincides with the axis of the piston and the centre of the tie member, and the body is arranged to contact the centre of the piston and of the tie member.

10. An assembly according to any of claims 1 to 9, in which each piston and stem combination is a one-piece moulding completed by a disc which is captive on the piston and, in conjunction with a step in the piston, defines a groove for receiving a ring seal.

11. An assembly according to any of claims 1, 2, 4, 7, 8 and 9, in which the body is an assembly of laminations each with a longitudinal edge with serrations of rack form.

12. An assembly according to claim 1, substantially as described with reference to Figures 1 and 2, Figure 3, Figure 4, Figures 5 to 9, Figures 10 and 11, or Figures 12 and 13 of the accompanying drawings.