GB2463052A - High pressure expanding hydraulic cylinder without sliding seals - Google Patents

Abstract

An axially extendable and compressible cylinder able to withstand high pressure differentials comprising a frusto-conical spiral wound laminate of alternate reinforcing 3 and elastomer 4 layers, where the axial deformation is enabled by shear in the elastomer layers 4 and the hoop strength necessary to withstand the pressure is provided by the reinforcing layer 3. The elastomeric layer 4 may be formed using injection moulding or casting; and the reinforcing member 3 may be made from a winding of a composite pre-preg or from flat sheet metal. A radial elastomeric limited angle journal (figure 6) is also disclosed.

Description

INTELLECTUAL

. .... PROPERTY OFFICE Application No. GB08 15804.0 RTM Date:24 February 2009 The following terms are registered trademarks and should be read as such wherever they occur in this document: Bellofram Corporation Festo Corporation Fluidic Muscle Kevlar Slinky Intellectual Property Office is an operating name of the Patent Office www.ipo.gov.uk Page 1

PRESSURE RESISTANT EXPANDING VOLUME CYLJNDER

The present application pertains to a novel high pressure hydraulic cylinder that can change its length as a response to internal or external hydraulic or mechanical forces, without involving any sliding seals.

FIELD OF THE INVENTION

The present invention relates to a hydraulic actuator that expands in length to change its internal volume rather than requiring a moving piston. By this means sliding seals become unnecessary, enhancing the units robustness as there is no risk of seal failure even when using contaminated fluids. Also sticton is avoided resulting in smooth

repeatable motion.

BACKGROUND OF THE INVENTION

Hydraulic cylinders have become a key engineering component. They all share the same need to create a pressure proof seal between the piston and the cylinder wall, so as to trap the high pressure fluid on one side. This seal has to be able to sustain the compressive loads while still being able to slide up and down the cylinder. Not surprisingly the seal is often the weak link and will need frequent service intervals.

In the 1950's Bellofram Corporation invented and developed a rolling diaphragm seal. It is commonly shaped like a top hat, where the top is inverted and folded in on itself around the piston and the outer side attaches to the cylinder. The fabric supporting the elastomer is woven so as to resist axial stretch but permit circumferential stretch, thereby enabling a smooth rolling action in the gap between the piston and cylinder. A limitation of this solution is that some compressive distortion of the elastomer is inevitable, resulting in wrinkling that can generate wear. A further limitation is the low safe maximum cycle rate of about I Hz. A further limitation is the need to maintain a positive pressure differential to keep the seal under stretch, which also precludes its use where both pressure and suction is required.

Other solutions are available that are based on the inflation of a style of bladder. These result in a crease around the edge of the load carrying pad where it inverts the bladder.

This increases the wear and precludes its use for high pressure applications.

Page 2 A further style of expanding volume actuator is avaabIe from Festo Corporation called the Fluidic Muscle. It works by wrapping an elastomeric tube with reinforcing fibers in a rhomboidal fashion such that when pressurized it expands circumferentially and in doing so contracts axiay. It is thereby limited to applying tension rather than compression which may complicate the mechanism. The musde' is limited in its maximum pressure and so is more suitable for pneumatic applications.

For very low pressure applications bellows type cylinders have been implemented.

These will tend to buckle under pressure, have undesirable internal flow characteristics, have a large surface area that needs restraining compared to their volume, exhibit tensile force concentrations and again cannot provide much suction.

OBJECTS OF THE INVENTION

A principal object of this invention, therefore, is to provide a new means to apply hydraulic or pneumatic compression without requiring sliding seals.

A further object of this invention is to act as a limited rotation journal which does not necessitate sliding seals.

A further object of this invention is to enable expansion or rotation to occur at high pressure differentials, at high cycle rates and providing long life.

Other and further objects will be explained hereinafter and more particularly delineated in the appended claims.

SUMMARY OF THE INVENTION

In summary, the invention proposes a new style of hydraulic cylinder that uses axial expansion of the cylinder itself when under pressure as the motive force. It is based on the use of multiple alternate layers of soft elastomer and rigid support material, so arranged as to optimize the potential shear along the axis to be expanded while resisting circumferential expansion.

The principal is that a soft elastomer is necessary for shear, but if the layer is too thick or not wide enough it could be blown out by the pressure differential. So by having multiple thin layers of elastomer enough shear can be provided for, and by supporting each layer independently they can be held in place.

Page 3 If the elastomer and supports were frusto-conical rings, then in order for the stack to expand, enough elastomer would have to migrate in from the edges to enable the increased separation. This is a bit like a small cross sectional area trying to fill a large cross sectional area void. The wider gaps will then be more prone to seal blow out.

So this invention proposes that the interleaving layers are wrapped around each other in a continuous frusto-conical spiraL The spiral has a tendency to puU out flatter when stretched, effectively reducing the wa thickness as a response to the greater axial length. This cooperates with the shear potential of the elastomer layers enabling them to deform to the new requirement.

If the conical vertex angle is sma then the assembly will have a high pitch, and with fewer turns per unit length will have less stretch, however the wa thickness will also be less. If the vertex angle is large then stretch becomes more difficult as the behavior tends towards that of the previously described rings.

The spiral layers may be fabricated from flat sheet by cutting out arc strips of the appropriate width and radius.

The relationship that defines the necessary arc radius (r) is: r = w/ ((pl/p2) -1) where w width of strip p1 = outer edge length = Pi (OD) / cos (vertex angle/2) p2 inner edge length P1 (ID) / cos (vertex angle/2) For example to enable wrapping around a 40 degree vertex cone angle with a 40mm ID, 4mm pitch and 8mm wall a strip could be cut to provide 2.4 revolutions.

These strip members then need to be joined, which if made in the preferred steel would be done by automated welding. If injection moulded out of a suitably stiff plastic then they can be made in a thin continuous flat spiral in the same manner as a slinky', with subsequent winding up and heat forming into the required frusto-conical spiral.

The interleaving elastomer layers may be bonded against the support layers in a final assembly tool that compresses the assembly together. It doesn't matter if the elastomer has periodic breaks as these will have little impact on the shear characteristics.

Page 4 Alternatively the elastomer can be co-moulded around the support spiral and between its end fittings. In this embodiment the reinforcing spiral is inserted into the moulding tool, with its outside edge located in conforming grooves in the tool. When moulded, the ID of the elastomer is smaller than the ID of the reinforcing, thereby providing a flow path for the injected plastic and resulting in a smooth internal bore.

If co-moulded, and steel reinforcing is desirable, instead of edge welding the arc segments together they could be put back to back and spot welded at alternate ends making a continuous spiral alternating between left had and right hand rotation. This facilitates additional shear in that pulling apart will tend to unwind the spiral, while pushing together will tend to wind it up. This effect though is minimal as the whole segment has can only shear as a small proportion of the elastomer thickness over its entire length. In the previous example, if the maximum shear available around 2.4 revs is 0.5mm (elastomer thickness of 1 mm) at a mean diameter of 48mm the induced rotation will be <0.5 degrees.

A further method to provide for frusto-conical spiral reinforcing is to weave a strong fibre such as polyester or Kevlar into a spiral band with a suitable preponderance of axial fibers, laminate with the elastomer retaining one end against a frusto-conical end cap, spiral wrapping it around a removable mandrel, and then hot pressing.

A further method to provide for frusto-conical spiral reinforcing is to lay up pre-preg GRP or CAP ribbon of a type that permits axial stretch, preferably simultaneously winding it up with the interleaving elastomer layer around a frusto-conical end plug and removable mandrel, and then compressing the laminate to drive out any air pockets during curing.

A further method to provide for frusto-conical spiral reinforcing is to wind up a hot plastic section around a mandrel between the leaves of a master' steel continuous frusto-conical spiral with a non-stick coating. The sandwich is then heated and pressed. The formed plastic spiral can then be unwound from the tool like a threaded fastener.

The design parameters are selected to enable the functional requirements. The higher the expected pressure differential, the greater the strip width and/or the thinner the elastomer layer, with greater width also enabling a smaller cone vertex angle (trading off axial stretch for hoop strength).

Page 5 Material characteristics wi also define performance. The ideal reinforcing spiral would have minimum thickness, maximum tensile strength and maximum elastic shape recovery. Probably the best performing options would be spring steel and CRP.

The deal elastomer layer will be soft and perfectly elastic so as not to heat up on rapid cycling. It would have good adhesion to the reinforcing spiral. The design for a given application would consider the maximum pressure differential, layer thickness, its depth (between its adjacent reinforcing), its Shore hardness and its target maximum shear.

Total axial extension is a product of the elastomer thickness, the number of layers along the length, the maximum shear allowable per layer and is affected to a degree by the cone angle and spiral pitch angle -the deformation of the support spiral under stretch being best accommodated when both are small. In practice, a suitably soft polyurethane or silicon based elastomer can enable a shear greater than its own thickness without getting close to its elastic performance limits. Suitable elastomers will exhibit a safe elongation of > 300%, a shear displacement equal to the thickness requires an elongation of only 141% (sqr2). At this shear degree, the described embodiment with revs would have an axial stretch capability of +-20 mm about its relaxed length of 80 mm. In other words this system may yield a safe length variation of about half its relaxed length.

A potentially useful attribute of this expanding cylinder invention is that the cylinder can also enable some rotation between the end faces. The degree of rotation will be defined by the number of revs in the stack times the shear available at each rev. Some change in overall diameter will occur as it reduces when winding up and increase when unwinding. This can be mitigated by employing coupled alternately left hand turn and right hand turn parts.

If only rotation is required, a simpler embodiment sees the spiral replaced by separate frusto-conical rings, again interleaved with elastomer layers. The total rotation will depend on the number of layers and the shear angle available in each layer at its OD. In the 80 mm OD embodiment shown, there are 20 layers providing +-20 mm of stretch around the perimeter which translates to a possible angle of rotation about the axis of about 60 degrees. The frusto-conical form is preferred to a flat disc as it provides for self regulated centering about the rotation axis.

Page 6 Best mode and preferred designs and techniques w now be described.

DRAWINGS

The present invention can best be understood in conjunction with the accompanying drawing, in which: Ag. 1 shows a side view of an expanding cylinder unit.

Ag. 2' shows a similar cylinder in cross section.

Fig. 3 shows a variation of the cylinder where the elastomer has been co-moulded into the spaces between the reinforcing spiraL Fig. 4 shows the cylinder as in Fig. 2 back to back with a counter-rotating spiral cylinder.

Fig. 5 shows a sectional view of a cylinder segment open at both ends, and comprising of two coupled counter-rotating spiral segments Fig. 6 shows an embodiment of a radial only elastomeric limited angle rotation journal.

In the drawings, preferred embodiments of the invention are illustrated by way of example, it being expressly understood that the description and drawings are only for the purpose of illustration and preferred designs, and are not intended as a definition of the limits of the invention.

PREFERRED EMBODIMENT(S) OF THE INVENTION Fig. 1 shows the first embodiment of the invention where 1 is a closed end cap, 2 is an open end that can be attached to a mechanism, 3 is the reinforcing frusto-conical spiral and 4 is the elastomeric infill spiral layer.

Fig. 2 shows the same embodiment with a different coupling part 5. The frusto-conical spiral has a vertex angle of 40 degrees, ID of 40 mm, OD of 56 mm, reinforcing thickness of 0.25 mm, elastomer thickness of 1 mm and spiral pitch of 4mm and 20 revs.

Page 7 This should enable a change in axial length from 60 mm in its compressed state to 100 mm in its tension state.

Fig. 3 shows a sectional view of a variant with a different end plug 7 and a co-moulded elastomer infiU 6. To manufacture this part, a 2 part injection moulding tool is prepared to retain the end cap and plug, with conforming spiral grooves to locate the reinforcing spiral (first wound up and held together on a removable transference jig). When the tool shuts the reinforcing ring and other parts become trapped, moRen elastomer can then be injected under pressure or cast in under vacuum.

In Fig. 4 the embodiment as shown in Fig. 2 has counter-rotating laminates 9 and 10 connected by the coupling part 8 (shown in section on Fig. 2 as 5) between end caps 1 and 11. Part 8 is aowed to rotate a little as the elastomer shears, making it a little easier for the laminate tube to change diameter to better facilitate stretch and compression, and because the winding torques from both ends cooperate they are relieved by this displacement so avoid any torque on the end fittings.

Fig. 5 shows that it is possible to couple counter-rotating laminates 12 and 13 without requiring a mirror image coupling part as in Fig. 2 5. To limit the circumferential expansion a conforming frusto-conical ring could be added at this interface.

A purely radial embodiment is described in Fig. 6. Here the spirals have been replaced by a series of reinforcing pressings 16 similar in shape to disc springs but more conical, with either separately introduced or co-moulded elastomer infill 17, all clamped between an end cap 14 and plug 15. In use these will be bolted to the connecting mechanism using the available flanges.

Further modifications of the invention will also occur to persons skilled in the art, and all such are deemed to fall within the spirit and scope of the invention as defined by the appended claims.

Claims (8)

1. Page 8CLAIMSWhat is claimed is: 1. An axially expanding hydraulic cylinder comprising a reinforcing layer interleaved with an elastomeric layer wound into a frusto-conical spiral and sealed at either end by either open or closed plug or cap fittings.
2. 2. The system of claim 1 where the elastomeric layer is introduced by co-moulding it around the reinforcing layer and between the end parts using either injection moulding or casting.
3. 3. A system as of claims 1 and 2 where the reinforcing layer is injection moulded in a flat spiral state like a slinky', prior to being wound up into the frusto-conical spiral.
4. 4. A system as of claims 1 and 2 where the reinforcing layer is produced from a composite pre-preg by winding it alongside the elastomeric layer, first retaining it against a frusto-conical end plug and guiding the wind over a mandrel, and then compressing the laminate between the plug and opposite end cap during curing.
5. 5. A system as of claims 1 and 2 where the reinforcing layer is produced from flat sheet metal welded or bonded into a continuous spiral prior to being wound up into the frusto-conical spiral.
6. 6. A system as of claims 1 and 2 where the cylinder is comprised of alternate clockwise and counterclockwise rotating laminate spiral parts, either bonded to each other or to intermediate coupling parts.

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7. 7. A radial elastomeric limited angle journal comprising of alternate frusto-conical reinforcing rings with inter'eaving eastomer sayers, bonded between an end cap and an end plug.
8. 8. A system as n daim 7 where the eastomeric layers are co-mouded into the gaps between the reinforcing layers.