Patent Application
20140262626
Not applicable.
This disclosure relates to improvements in pistons for lift cylinders, such as the lift cylinders in a mast assembly for a material handling vehicle.
Vehicles, such as material handling vehicles, are used to hoist and carry materials such as, for example, pallets or crates through a factory or a warehouse. One common material handling vehicle is a lift truck. Lift trucks typically have an upright telescopic mast attached to the forward end of the truck. The mast actuates a carriage, or forks, on which supporting materials can be lifted by the upward extension of the telescopic mast.
Such lift trucks often operate in tight spaces and are expected to safely lift objects to great heights. However, there are design trade-offs between providing short overall truck length and providing the high elevating heights.
Hence, there is a continuing need for mast assemblies that are capable of greater extension without significant modification to other parts of the vehicle.
To date, the design of pistons for lift cylinders has been based on meeting the pressure or flow requirements of the hydraulic system. Accordingly, the pistons of conventional lift cylinders have been constructed of homogenous, isotropic materials such as standard and high strength steels. This reliance on high strength steels as construction materials for lift cylinders has led those in the industry to ignore the other limitations created by the use of these materials.
In this disclosure, modifications are proposed to the material structure of a piston for a lift cylinder that improves the buckling strength of the piston. This addresses a problem ignored by existing piston design and further enables an increased extension length for the lift cylinder without failure of the piston due to the piston exceeding a critical buckling strength.
An improved lift cylinder for a mast assembly of a vehicle, such as a material handling vehicle, is disclosed. The lift cylinder includes a cylinder housing with a piston received therein. The piston is movable along an axis of extension between an extended position and a retracted position. The piston comprises a composite material having a reinforcement phase oriented along the axis of extension, thereby increasing the buckling resistance of the piston (relative to a similar piston lacking the axially-oriented reinforcement phase).
The lift cylinder may be based on any of a number of different structural designs and may implement any of a number of piston types. For example, the piston may be a hollow piston or a solid piston.
The composite material may include a matrix material with the reinforcement phase dispersed therein such as, for example, a metal matrix composite material. In some forms, the matrix material may be a ferrous material including, for example, ferrous alloys, steel, or steel alloys including high strength steels. In one form, the reinforcement material may be a graphite-epoxy sub-matrix centrally disposed in the steel. In another form, the reinforcement phase may comprise tungsten carbide.
It is contemplated that in some forms, the reinforcement phase may be continuous from a first axial end of the piston to a second axial end of the piston using strands or other columns of the reinforcement material. In other forms, the reinforcement phase may be discontinuous from a first axial end of the piston to a second axial end of the piston (e.g., discontinuous whiskers or fibers).
The piston may include a sealing surface. In some instances, the reinforcement phase might provide discontinuous surface properties and pose problems in forming a reliable seal. Accordingly, a coating, such as a chrome-plating, may be applied over the composite material to provide the sealing surface.
It is contemplated that a lift cylinder of the types described above (and further described below) may be included in a vehicle. Thus, according to another aspect, a vehicle is disclosed including a mast assembly having one or more lift cylinders as recited herein. The mast assembly includes multiple sections that are extendable relative to one another via extension of the lift cylinder(s). The vehicle may be a lift truck, although other types of vehicles could also employ the improved lift cylinder with the modified piston.
By providing a piston having anisotropic properties in the direction of piston extension through use of an oriented reinforcement phase, the piston is provided with improved buckling resistance. Among other things, with this improved piston a lift cylinder can extend a greater distance (i.e., a mast assembly may be extended to greater heights by greater extension of the lift cylinder) without concern that the lift cylinder and piston, specifically, will fail as the result of buckling. Although the improvement of the material properties of the piston in the direction of extension comes at a cost of the materials properties in the directions perpendicular to the direction of extension, because the materials properties in these other directions are rarely design limiting, their compromise or reduction is tenable.
Moreover, even for systems in which an increase in extension length would not necessarily be advantageous, an improvement in buckling strength permits the use of higher pressures with smaller-sized lift cylinders. This reduction in lift cylinder size (e.g., diameter) means that the overall package size or footprint of the vehicle can be reduced while maintaining the original extension length.
These and still other advantages of the invention will be apparent from the detailed description and drawings. What follows is merely a description of some preferred embodiments of the present invention. To assess the full scope of the invention the claims should be looked to as these preferred embodiments are not intended to be the only embodiments within the scope of the claims.
Referring first to
Now with additional reference to
The base section 126 is comprised of a pair of spaced, base rail members 132 and 134 connected together at their bottom ends by a base crosstie 136 and at their upper ends by a pair of crossties 138 and 140. The crossties 138 and 140 include a set of louvers which help to provide structural rigidity and bending resistance. The crosstie 140 may also serve to support a protective guard 142, as is illustrated in
The telescopic section 128 is comprised of a pair of spaced, upright mid rails 144 and 146 connected at their lower ends by a lower crosstie 148. An upper crosstie 150 extends rearward from the upper ends of the mid rails 144 and 146 and then laterally across the space between the mid rails 144 and 146 to maintain their parallel alignment. The rearward extending portions of the crosstie 150 also provide a connection point for a pair of main lift cylinders to be described in more detail below.
The inner telescopic section 130 is likewise comprised of a pair of spaced, upright top rails 152 and 154 connected at their lower ends by a lower crosstie 156 and connected at their upper ends by an upper crosstie 158.
Notably, the telescopic mast structure is raised and lowered by a pair of lift cylinders 172 and 174. The lower ends of the lift cylinders 172 and 174 are fastened to the base section 126 adjacent each end of the base crosstie 136. Piston rods 176 and 178 extend upward from respective main lift cylinders 172 and 174 and fasten to the upper crosstie 150 on the outer telescopic section 128. When the lift cylinders 172 and 174 are hydraulically operated in response to commands from the operator, the outer telescopic section 128 is lifted and lowered with respect to the base section 126 to extend and retract the mast 122.
Turning now to
A hydraulic chamber 226 is defined in the piston 214 and the cylinder housing 212 which includes a bore section of the hollow body of the piston 214. Between the radially outward facing sealing surface 228 of the piston 214 and a seal gland 230 disposed at the head end of the cylinder housing 212, a series of seals 232 are provided in order to help prevent the leakage of any fluids or oils from the lift cylinder 210. Hydraulic fluid can be pumped into the hydraulic chamber 226 in order to move the piston 214 relative to the cylinder housing 212 (the point of hydraulic fluid entry is not shown in the illustration of
Those having ordinary skill in the art will appreciate that this is but one exemplary type of piston design for one exemplary type of lift cylinder for one exemplary type of material handling vehicle and that the modifications to the piston structure disclosed herein would be equally applicable to other pistons for other lift cylinders that may be used in other types of vehicles. Accordingly, the vehicles, lift cylinders, and piston types are intended to be illustrative, but not limiting. For example, while a material handling vehicle in the form of a lift truck is illustrated, the disclosed and improved lift cylinder may be employed in other types of vehicles other than material handling vehicles. As still another example of a modification that may be made from the illustrated embodiments, it is contemplated that pistons involving this invention might have seal-to-bore designs and seal-to-rod designs.
To increase the buckling resistance of a piston in such a lift cylinder, the piston may be formed from a composite material having a reinforcement phase oriented along the axis of extension. Such an engineered composite material can increase the strength and stiffness of the piston in the axial direction.
In some forms of the composite, a ferrous continuous phase (the matrix) is combined with an oriented higher strength discontinuous phase (the reinforcement phase) in order to increase the modulus of elasticity or other materials properties in the axial direction of the finished piston component. The materials properties in this axial direction will be referred to by the term “E1” in this document. Incorporation of an anisotropic material increases resistance to axial loading along the long axis of the lift cylinder (i.e., the axis of extension), while still maintaining sufficient resistance to bending and transverse loadings in directions perpendicular to the loading/extension direction.
To understand how and why such direction modification of the material improves buckling resistance, the following formulas and explanation are offered. Euler's formula for critical buckling resistance of a slender column is
where FCR is the critical load allowable, E is the modulus of elasticity, I is the moment of inertia, K is a factor accounting for the end conditions (e.g., are the ends fixed, rounded, or pivoted), and L is the length of the column.
For a column of solid circular cross section (such as a solid piston), the formula for moment of inertia is
where D is the diameter of the circular cross section. To calculate the moment of inertia for a hollow cylindrical cross section (such as a hollow piston), the D4 term is changed to (Do4-DI4), where Do is the cylinder outside diameter and DI is the cylinder inside diameter.
Accordingly, returning to Euler's formula for critical buckling, for a lift cylinder column in which L and I are constant, increasing E proportionately increases FCR. Given constant size and length constraints, this formula shows that increasing E in the direction of axial extension or loading will also increase the critical buckling strength resistance.
In order to increase E in this direction of axial extension or loading, a second reinforcement phase is introduced into the base or matrix material. In matrix-fiber composites in which the fibers are parallel with one another, the rule of mixtures for materials dictates that:
E
1
=V
f
E
f
+V
m
E
m
where E1 is a material property of the composite material in the direction parallel to the fibers (that is the primary direction of loading), Vf is the volume fraction of the fibers, Ef is the material property of the fibers, Vm is the volume fraction of the matrix, and Em is the material property of the matrix. This equation can be used to provide an estimation of how the volume breakdown of the reinforcement fibers in the matrix affects the overall material properties of the material.
Because typical material handling equipment lift cylinder mounting results in floating housing designs that eliminate loading in the transverse or bending direction, increasing E1 material properties may be achieved without severely compromising the E2 and E3 material properties (which are perpendicular to one another and to the E1 loading direction). Put another way, any loss or degradation of the properties in the E2 and E3 directions as the result of preferentially increasing the properties in the E1 direction are trivial and generally acceptable, since the E2 and E3 materials properties are not severely taxed under the applied loads.
Creation of this anisotropic composite material for the piston might be done in a number of ways which are now outlined. These exemplary embodiments of anisotropic composite materials are intended to be illustrative, but not limiting. Accordingly, other design geometries (i.e. piston or phase shapes or geometries or phase distributions) may be employed as well as other materials offering comparable properties.
First, with reference to
While the outer circumference 318 may provide the sealing surface and not need to be modified, it is noted that, in some lift cylinder configurations or constructions, the inner circumference 320 of the bore may be a sealing surface. In this instance, it is contemplated that the surface may be coated or treated to provide the continuous surface qualities in order to establish and maintain a seal-to-bore interface if the reinforcement phase does not provide appropriate sealing qualities.
Moreover, it is contemplated that by redistributing the reinforcement phase to the outside (e.g., closer to the outer circumference 318) or the outside surface, the inside surface being steel to maximize E2 and E3 properties, the buckling resistance might be further optimized within the cross-sectional area available.
Likewise it is contemplated that the piston might be modified to be an MMC (metal matrix composite) material, with axial placement of high strength material reinforcement within the ferrous or steel matrix of the piston. It is contemplated that the reinforcement phase might be axially discontinuous (i.e., whiskers or fibers of reinforcing material that are generally oriented along the extension and loading axis, but that do not extend end-to-end in the piston) or might be continuous (i.e., strands or rods implanted within the matrix material that substantially extend from one end to the other end of the piston). In various forms, the matrix material may be a ferrous, steel, or high strength steel and the reinforcement phase may be a non-ferrous material or materials to resist loading in the axial direction.
As another example and with reference to
In such an arrangement as illustrated in
Accordingly, various benefits may be derived from modification of the piston to be formed from a composite material with anisotropic materials properties. Among other things, the piston may be improved to provide greater unsupported extension length. Moreover, improving the axial properties of the piston allows systems designers to migrate toward the use of higher pressure systems (such as 5000 psi systems which are becoming more common in the industry), allowing higher loading capacities, without increasing the diameter of the piston. Likewise, existing systems might be made with smaller diameter pistons and higher pressures to handle the same loadings, therefore, smaller form lift cylinders which permit smaller headlength vehicles, machine packaging, and so forth in which the size reduction would otherwise present problems with buckling failure of the piston.
It should be appreciated that various other modifications and variations to the preferred embodiments can be made within the spirit and scope of the invention. Therefore, the invention should not be limited to the described embodiments. To ascertain the full scope of the invention, the following claims should be referenced.
