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(11) | EP 0 040 976 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Rotary actuators |
| (57) A piston-rack rotary fluid pressure actuator includes a housing (10) enclosing cylinder
(11) in which is disposed a pair of pistons (12, 13) biased by springs (14 and 15)
towards the centre of the cylinder. Each piston has a rack portion (16, 17) engaged
with a pinion (19) on output shaft (18), and elements of low friction material (20,
21) are incorporated into the outer surface of the rack portions (16, 17) to take
up forces tending to displace the rack-bearing portions towards the cylinder. Annuli
(30, 31) of incompressible material keyed to the shaft (18) make rolling contact with
runways (32, 33) on the rack-bearing portions and have diameters equal to that of
the pitch circle diameter of the teeth of pinion (19). By this means contact is maintained
between the low-friction elements (20, 21) and the cylinder and over-meshing between
the teeth of pinion (19) and racks (16, 17) is prevented. |
[0002] A disadvantage of a piston-rack rotary actuator which includes a pair of pistons which are displaceable towards and away from one another along a pair of opposed cylinders is that the pistons have a tendency to cock or turn in the cylinders thereby jamming and/or damaging the radial inner surface or surfaces of the cylinders and the seals.
[0003] In UK Patent 1,251,805 this was overcome by providing guide members to constrain the piston to move only in an axial direction, the piston sliding on stationary guide members. However, in some designs, it is desirable to have a large pinion on which the racked piston acts and it is not possible conveniently to provide guide members, such as described in UK Patent 1,251,805. A solution is proposed in US Patent 4,167,897 where a rack bearing portion of each piston is provided with low coefficient friction elements between that portion and an internal surface of a cylinder to prevent scoring and to maintain the rack in contact with a pinion of the actuator. In the US specification, annuli are also provided of low friction elastomeric or synthetic resin material at each side of the pinion which slide on runways provided on each side of the rack portions. Such annuli are provided to tend to urge the racked portions of the pistons towards the low friction element referred to above.
[0005] Acc.ording to the invention there is provided a piston rack rotary fluid pressure actuator, comprising a housing including a cylinder having a central longitudinal axis; a piston disposed coaxially within said cylinder and arranged for linear movement substantially parallel to said central longitudinal axis of said cylinder under the influence of a difference in fluid pressure created on opposite sides of said piston, said piston including a rack-bearing portion having a toothed rack rigidly connected to said piston; an actuator output shaft rotatably mounted in said housing and disposed transverse to said central longitudinal axis of said cylinder; a pinion rigidly affixed to said actuator output shaft and disposed so that teeth of the pinion engage with teeth of the rack; at least one element having a low coefficient of friction being disposed between and in contact with said rack-bearing portion of said piston and the adjoining portion of the internal surface of said cylinder whereby any forces which tend to displace said rack-bearing portion of said piston toward said adjoining portion of said internal surface of said cylinder are counteracted, characterised by spaced annuli of non-compressible material rigidly fixed for rotation with said actuator output shaft in the vicinities of the opposite ends of said teeth of said pinion, said annuli having diameters equal to the pitch circle diameter of said teeth of said pinion, two spaced flat runways provided on said rack-bearing portion of said piston; and said annuli forming a rolling contact with the runways on the piston during operation of the actuator to help to maintain contact between each said low friction element and the internal surface of said cylinder, to prevent rotational movement of said piston, and to prevent over-meshing of the teeth of the rack and the pinion.
[0006] According to another aspect of the invention there is provided a housing including a cylinder having a central longitudinal axis; two pistons disposed coaxially within said cylinder and arranged for linear movements substantially parallel to said central longitudinal axis of said cylinder either towards or away from one another under the influence of a difference in fluid pressure created on opposite sides of each of said pistons, each said piston including a rack-bearing portion having a toothed rack rigidly connected to said piston; an actuator output shaft rotatably mounted in said housing and disposed transverse to said central longitudinal axis of said cylinder at the mid-length point of said cylinder; a pinion rigidly affixed to said actuator output shaft and disposed between said racks integral with said pistons so that teeth of the pinion engage with teeth of the racks; at least one element having a low coefficient of friction being disposed between and in contact with both said rack-bearing portion of each piston and the respective portion of the internal surface of said cylinder whereby any forces which tend to displace said rack-bearing portion of each of said pistons toward said respective portion of said internal surface of said cylinder are counteracted, spaced annuli of non-compressible material rigidly fixed for rotation with said actuator output shaft in the vicinities of the opposite ends of said teeth of said pinion, said annuli having diameters equal to the pitch circle diameter of said teeth of said pinion, two spaced flat runways provided on said rack-bearing portion of each of said pistons; and said annuli forming a rolling contact with the runways on the pistons during operation of the actuator to help to maintain contact between each said element and the internal surface of said cylinder to prevent rotational movement of said pistons, and to prevent over-meshing of the teeth of the racks and the pinion.
[0007] A piston rack rotary fluid pressure actuator according to the invention will now be described by way of example, with reference to the accompanying schematic drawing, in which:
Figure 1 is a cut-away side elevation of the actuator;
Figure 2 is a section through A-A of Figure 1; and
Figure 3 is an isometric view of one piston and a pinion of the actuator.
[0008] Referring to the drawings, in Figure 1 a housing 10 encloses a cylinder 11 in which two pistons 12 and 13 are disposed coaxially within the cylinder 11 and spring biased by springs 14 and 15 towards the centre of the cylinder 11. Each piston has a toothed rack portion 16 and 17. An output shaft 18 rotatably mounted in the housing 11 has a toothed pinion 19 keyed to the shaft 18 which engages the teeth of both the racks 16 and 17. Low friction bearing material 21 such as DELRIN (Trade Mark) is supported in grooves 20 in outer surfaces of the rack portions 16 and 17. Similarly, low friction material 22a, formed as trapezoids, is supported in grooves 22 in each piston 12 and 13.
[0009] Spaced annuli 30 and 31 (best seen in Figure 3) keyed to the shaft 18 and having a diameter equal to the pitch circle diameter of the teeth of the pinion 19 are mounted at opposite ends of those teeth. Two flat runways 32 and 33 are fixed to each of the rack bearing portions 16 and 17 to contact the annuli 30 and 31 respectively when the teeth of the pinion 19 and the teeth of the racked portions 16 and 17 are in correct meshing position.
[0010] In operation, pressurised air or liquid is supplied into the cylinder 11 between the pistons 12 and 13 to force the pistons 12 and 13 away from each other and the centre of the cylinder 11 and so rotate the shaft 18. When the pressure is reduced the springs 14 and 15 return the pistons towards their central positions. During movement of the pistons 12 and 13, the annuli 30 and 31 rotate with the shaft 18 to form rolling contacts with the runways 32 and 33. This contact maintains the contacts between the material in the grooves 20 and the walls of the cylinder 11 and at the same time prevents overmeshing of the teeth of the pinion 19 and the teeth of the rack portions 16 and 17, and rotation of the pistons 12 and 13.
[0011] By forming the annuli 30 and 31 and the runways 32 and 33 of load bearing material such as anodised aluminium, or say, steel where corrosion is not a problem, there is no tendency for such parts to be compressed or deformed by any forces acting transversely to the axis of the cylinder 11. Further, by arranging for the contact between the annuli 30 and 31 and the runways 32 and 33 to be a rolling contact any tendency for the annuli to stick or slide is overcome.
[0012] In a prior art proposal, in US Patent 4,167,897, the material used for the annuli being elastomeric or synthetic material there is a tendency for the material to become deformed in use. Further, as the contact is not a rolling contact, both the annuli or sometimes one annulus tends to jam and cause uneven wear which means that excessive wear occurs on the teeth due to overmeshing at one or both sides of the pinion 19 or rack bearing portions.
1. A piston rack rotary fluid pressure actuator, comprising: a housing including a
cylinder having a central longitudinal axis; a piston disposed coaxially within said
cylinder and arranged for linear movement substantially parallel to said central longitudinal
axis of said cylinder under the influence of a difference in fluid pressure created
on opposite sides of said piston, said piston including a rack-bearing portion having
a toothed rack rigidly connected to said piston; an actuator output shaft rotatably
mounted in said housing and disposed transverse to said central longitudinal axis
of said cylinder; a pinion rigidly affixed to said actuator output shaft and disposed
so that teeth of the pinion engage with teeth of the rack; at least one element having
a low coefficient of friction being disposed between and in contact with said rack-bearing
portion of said piston and the adjoining portion of the internal surface of said cylinder
whereby any forces which tend to displace said rack-bearing portion of said piston
toward said adjoining portion of said internal surface of said cylinder are counteracted,
characterised by spaced annuli (30, 31) of non-compressible material rigidly fixed
for rotation with said actuator output shaft (18) in the vicinities of the opposite
ends of said teeth of said pinion (19), said annuli having diameters equal to the
pitch circle diameter of said teeth of said pinion, two spaced flat runways (32, 33)
provided on said rack-bearing portion of said piston (13); and said annuli (30, 31)
forming a rolling contact with the runways (32, 33) on the piston during operation
of the actuator to help to maintain contact between each said low friction element
and the internal surface of said cylinder, to prevent rotational movement of said
piston (13), and to prevent over-meshing of the teeth of the rack (17) and the pinion
(19).
2. A piston rack rotary fluid pressure actuator, comprising: a housing including a
cylinder having a central longitudinal axis; two pistons disposed coaxially within
said cylinder and arranged for linear movements substantially parallel to said central
longitudinal axis of said cylinder either towards or away from one another under the
influence of a difference in fluid pressure created on opposite sides of each of said
pistons, each said piston including a rack-bearing portion having a toothed rack rigidly
connected to said piston; an actuator output shaft rotatably mounted in said housing
and disposed transverse to said central longitudinal axis of said cylinder at the
mid-length point of said cylinder; a pinion rigidly affixed to said actuator output
shaft and disposed between said racks integral with said pistons so that teeth of
the pinion engage with teeth of the racks; at least one element having a low coefficient
of friction being disposed between and in contact with both said rack-bearing portion
of each piston and the respective portion of the internal surface of said cylinder
whereby any forces which tend to displace said rack-bearing portion of each of said
pistons toward said respective portion of said internal surface of said cylinder are
counteracted, characterised by spaced annuli (30, 31) of non-compressible material
rigidly fixed for rotation with said actuator output shaft 18 in the vicinities of
the opposite ends of said teeth of said pinion (19), said annuli having diameters
equal to the pitch circle diameter of said teeth of said pinion, two spaced flat runways
(32, 33) provided on said rack-bearing portion of each of said pistons (12, 13); and
said annuli (30, 31) forming a rolling contact with the runways (32, 33) on the pistons
during operation of the actuator to help to maintain contact between each said element
and the internal surface of said cylinder, to prevent rotational movement of said
pistons (12, 13), and to prevent over-meshing of the teeth of the racks (16, 17) and
the pinion (19).
3. A piston. rack rotary fluid presssure actuator according to Claim 2, in which said
annuli (30, 31) are formed of anodised aluminium.-
4. A piston rack rotary fluid pressure actuator according to Claims 2 or 3, in which
said runways (32, 33) are formed of anodised aluminium.
5. A piston rack rotary fluid pressure actuator according to any of Claims 2 to 4,
in which said teeth of said pinion (19) and said annuli (30, 31) are keyed to said
actuator output shaft (18).
6. A piston rack rotary fluid pressure actuator according to any of Claims 2 to 4,
in which said annuli (30, 31) are attached to said actuator output shaft (18) by locking
.screws.