When your cylinders need a little guidance
Benjamin Shobert, Polygon Co., Walkerton, Ind.
Specifying guide rod bushings involves evaluating performance factors, as well determining whether your system is better suited for conventional or composite materials.
Guide rod bushings are a necessary design component in most hydraulic and pneumatic applications. The selection criteria can vary widely, as can the family of material options available. Designers can select from traditional metallic powdered metal (PM) structure bearings, metal-backed bearings, thermoplastic materials, and composite bearings. Within each of these bearing families are a number of material and design choices that impact critical performance factors such as stick/slip, frictional response under varying load and motion conditions, and bushing wear. This article will focus on comparisons between conventional metallic bearing materials and what are commonly known as composite guide rod bushings.
Typical programs where metallic guide rod bushings are replaced are driven from one or several of the following factors:
* Improved stick/slip properties
* Significant reduction in shaft scoring
* Extension in bearing life
* Reduction in bearing profile
* Greatly improved side load or misalignment capacity
* Significant increase in load capacity of bearing.
Composite guide rod bushings are commonly available in two formats: a composite bearing utilizing a sintered PTFE liner or a composite bearing using a PTFE fabric liner. The most common composite guide rod bushing today is the sintered liner because of two primary performance enhancements over the fabric-lined bearing: frictional response under start-up conditions and transfer of PTFE to the wear surface.
Sintered (PM) Structure Bronze - Sintered powder metal structure bearings rely on an internal lubricant entrapped in the metallic structure as it undergoes the sintering process. As the bearing is cycled, the lubricant migrates to the wear surface, both as a natural function of relieved internal bearing stress which allows the lubricant to flow to the area of bearing wear, and as the bearing itself is worn away and the lubricant contacts the pin material. Several problems exist for this type of bearing material.
First, these bearings have a poor load capacity in either dynamic or static conditions. In linear slide block applications, the bearings wear to accommodate the emerging load pattern during the bearing's cycle. As this process advances, the bearing assembly's accommodation will translate into increased slop in the slide block itself, and will ultimately result in a slide block that is no longer cycling per the manufacturer's recommendation.
Second, sinter structure bronze bearings have a lubrication mechanism that is unreliable and easy to deplete, resulting in shaft scoring, high friction, and high wear. PM structure bearings must wear to continue to transfer lubricant to the wear surface. In linear slide applications, the surface area that must be covered with lubricant is significantly greater than what is seen in oscillatory or rotational movement environments. As such, the frictional response and wear patterns of PM structure bearings degrade more rapidly than higher performance bearing materials.
Metal-backed - This family of bearing materials is divided into two product types: the first is true ring structure metal-backed bearings and the second is split seam journal bearings. Ring structure bearings are becoming increasingly expensive to manufacture given the means by which the bearing liner is inserted into the bearing ID. The labor required to complete this process, as well as the necessary secondary labor to manufacture the bearing to the tolerances required result in an expensive bearing.
The second type of metal-backed bearing is the more common split seam journal bearing. This bearing exhibits good frictional response during start-up conditions but is prone to excessive wear. The PTFE overlay is thin (typically only 0.005 in.) and is quickly worn away in linear motion applications where the surface area the PTFE must be transferred to is fundamentally greater than the surface area of a conventional rotational or oscillatory application. In addition, initial application start-up running clearances change quickly in metal-backed bearings because of the thin, soft PTFE overlay on the bronze inter-structure being scrubbed off the bearing bore surface. Strict running clearances quickly disappear as the liner wears and tries to stabilize. Depending on shaft finishes, wear simply accelerates, resulting in unwanted clearances and assembly looseness.
Thermoplastics - A common and cost-effective guide rod bushing material is thermoplastic. These type of bearing materials share most of the design and performance limitations that PM structure metal bearings do because the thermoplastic bearing material itself is similar in its structure to that of a PM metallic bearing. However, the thermoplastics have two additional problems.
In applications where the slide velocity is high, a thermoplastic guide rod bushing does not tolerate the heat generated from such quick response requirements. The most common thermoplastic bearing grade materials will bind on the shaft and actually begin to break down mechanically as the bearing is cycled. The amount of lubricant and fillers will play a dynamic role in the relationship between mechanical and performance degradation as it relates to velocity.
Thermoplastic bearing materials are also prone to cold flow. Under constant load (even assuming in a theoretical application environment little to no load), many thermoplastic guide rod bushings will exhibit creep. This creep will result in slop in the bearing assembly and will most critically effect any pick-and-place precision the slide block is expected to maintain.
In some linear motion application environments, a black debris develops on the distal and proximal ends of the shaft during normal cycling conditions. This debris is commonly seen when two types of bearing materials are used: a sintered PTFE lined bearing and a sintered (PM) structure bronze or brass bearing.
This debris is most commonly the result of a complex interaction between the pin material itself, the liner selection, and the rate of deceleration of the bearing assembly. In some linear guide applications, the weight of the bearing assembly itself creates a macro-mechanical edge rolling condition as the assembly decelerates. For a sintered PTFE lined bearing, this deceleration causes parts of the bearing liner to roll as the motion reverses itself. The inherent potential for the resin the PTFE is entrapped within to be plasticine causes the resin to bind against the shaft. As this phenomena is repeated, the liner will fatigue and begin to transfer large macroscopic portions of the liner onto the shaft.
This debris deposition is application-specific and is not seen in all application environments. In other application environments, the black debris is seen in relation to sintered (PM) structure bronze or brass bearings. In this case, the black discoloration is not purely a deposition of material onto the shaft, but rather a scoring effect common to ring structure bearings that have a low tolerance for missing lubricant or contamination.
The solution to an application where liner debris is being deposited on the shaft is to alter the bearing's wear surface to a non-resinous and non-metallic liner. In these cases, transfer to a PTFE fabric lined bearings. These bearings incorporate high tenacity PTFE filaments in their continuous architecture. This is in contrast to PTFE resinous systems which rely on either a sintered powder form of the PTFE polymer or to another resin (such as acetal) with PTFE fibers randomly dispersed within the resin itself.
The composite PTFE-fabric bearings that have high tenacity PTFE filaments in their architecture allow for the bearing assembly to undergo aggressive deceleration conditions without depositing the PTFE or the resin carrier medium onto the shaft. This is because the wear surface of the fabric lined bearings utilize the filaments themselves without reliance on a resinous impregnation.