Great for linear motion
PolyLube® Composite Guide Rod Bearings are designed as straightforward replacements for traditional metallic materials. Replacement is typically driven from one or a combination of several of the following factors:
Reasons to Design with PolyLube® Guide Rod Bearings
- Improved stick-slip properties
- Optimal frictional response during cycling
- Significant reduction in shaft scoring
- Extension in the bearing life
- Reduction in bearing profile
- Greatly improved side load/misalignment capacity
- Increase in load capacity of bearing
- Enhanced corrosion resistance
- Tolerance of more cost effective shaft finishes
- Lower in weight
- Eliminates galvanic reactions
PolyLube Guide Rod Bearings are commonly available in two formats: a PolyLube bearing utilizing a glass liner or a PolyLube bearing utilizing a PTFE fabric liner. The most common PolyLube guide rod bearing in use today is the glass liner due to two primary performance enhancements over the PTFE fabric lined bearing: the frictional response under start-up conditions and the transfer of PTFE to the wear surface.
Comparisons to Common Guide Rod Bearing Materials
Sintered (PM) Structure Bronze
Sintered powder metal (PM) structure bearings rely on an internal lubricant that is entrapped into the metallic structure as it undergoes the sintering process. As the bearing is cycled the lubricant migrates to the wear surface as a natural function of compression and thermal expansion which forces the lubricant to flow to the area of bearing wear, but also as the bearing itself is worn away and the lubricant finds itself in contact with 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, this load capacity can become increasingly problematic. As the load on the bearing assembly increases, the bearings will 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 requirements as well as causing increased seal wear from piston misalignment.
Second, sinter structure bronze bearings have a lubrication mechanism that is both unreliable and easy to deplete. This means that shaft scoring, high friction, and high wear are all anticipated with these bearing materials. PM structure bearings must wear in order 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 much more rapidly than higher performance bearing materials.
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 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 overly 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 very thin (typically only 0.001" or 0.025mm) and is quickly worn away in linear motion applications where the surface area that the PTFE must be transferred to is greater than the surface area of a conventional rotational or oscillatory application. In addition, start-up running clearances change very quickly in metal-backed bearings due to the thin soft PTFE overlay on top of the bronze inter-structure being scrubbed off of the bearing bore surface. Strict running clearances quickly disappear as the liner wears and tries to stabilize. Depending upon shaft finishes, wear simply accelerates resulting in unwanted clearances and assembly looseness. A PolyLube® composite self-lubricating bearing offers minimal break-in and reliable self-lubrication through application life.
A common and low cost guide rod bearing material is thermoplastics. These types 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 as that of a PM metallic bearing. Thermoplastics however have two additional problems associated to linear motion environments.
First, in applications where the slide velocity is high, a thermoplastic guide rod bearing 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.
Second, thermoplastic bearing materials are prone to cold flow. Under constant load many thermoplastic guide rod bearings will exhibit creep. This creep will result in slop in the bearing assembly and will have a negative impact on any precision the slide block is expected to maintain.
Black Debris Shaft Deposition
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 a sintered PTFE lined bearing is used.
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 (not a fabric PTFE lined bearing), this deceleration causes parts of the bearing liner to roll as the motion reverses itself. The nature of the resin the PTFE is entrapped within can create the potential for the resin itself to bind against the shaft. As this phenomena is repeated, the liner will fatigue and begin to transfer macroscopic portions of the liner onto the shaft due to the thin soft PTFE overlay on top of the bronze inter-structure being scrubbed off of the bearing bore surface. Strict running clearances quickly disappear as the liner wears and tries to stabilize. Depending upon shaft finishes, wear simply accelerates resulting in unwanted clearances and assembly looseness. A PolyLube® composite self-lubricating bearing offers minimal break-in and reliable self-lubrication through application life. 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, Polygon recommends transfer to one of its PolyLube® fabric lined bearings such as the PolyLube® MRP 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 PolyLube 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.
PolyLube ID Seal Configurations
Incorporating T-lip wiper seals, radial shaft seals, o-rings or any other similar internal sealing system is not a problem for PolyLube Guide Rod Bearings. Polygon’s internal fabrication capabilities allow for easy and economical incorporation of ID features required to install common sealing systems.
Two liner thicknesses are available in the standard PolyLube PTFE tape lined bearing configuration: the 0.015" (0.38mm) liner being standard and a 0.030" (0.76mm) liner also being available for applications where seal geometry might require the introduction of a thicker liner to accommodate a unique ID feature. The 0.030” (0.76mm) liner can also be used in applications where boring the ID might be required in order to achieve tighter tolerances in an effort to address sizing and minor misalignment conditions.
PolyLube Fabrication Capabilities
One common fabrication detail seen on guide rod bearing applications deal with corner radiuses on internal and external grooves. Because Polygon uses a diamond wheel or groove tool to form the grooves we need to have at least a .015-.020" (0.38-0.51mm) corner radius. When threads are used there is usually clearance involved. When assembled with the mating part the bearing could shift to one side or the other impacting the location of the bearing surface in relation to the piston shaft. This could have a negative impact on wear.
The only other fabrication issue commonly seen on incoming prints is a surface finish called out on the internal diameter. This is typically related to an OEM’s historical use of machined bronze bearings in the application. Since the bronze is machined from a solid piece or casting, the surface finish is called out since it is related to the speeds and feeds of their fabrication process. The wear surface on PolyLube bearings is not machined so the surface finish call out can be removed from fabrication requirements.
Polygon is capable of holding a TIR I.D. to O.D. within .002" (0.05mm) and straight diameters to ±0.0005" (±0.013mm).
Mechanical and Physical Properties
PolyLube Guide Rod bearings are manufactured by a filament winding process that results in a continuous fiberglass filament backing ensuring excellent mechanical properties (especially fatigue resistance). The filament wound fiberglass structure uses a high strength, corrosion resistant epoxy resin as the matrix material. The high strength backing permits the use of a thin wall (1/16" to 1/8" or 1.5mm to 3.0mm) bearing which can often reduce the size and weight of the finished bearing assembly. This family of materials exhibits exceptional dimensional stability and performance predictability over wide temperature ranges (±325°F or ±163°C).
This bearing’s operating temperature range is ±325°F (±163°C). Maximum continuous operational surface temperature for the standard formulation is 325°F (163°C), depending upon load characteristics. The bearing has been heat stabilized at these temperatures, so that little dimensional change will occur in the bearing during operation. In a free state, the coefficient of expansion of the PolyLube® Guide Rod Bearing is 4.8x 10-6 in/in/°F (8.69 x 10-6 m/m/°C), lower than steel and most metals.
Guide Rod Bearing Properties
Glass Tape Bearings - Recommended for non-contaminating environments.
MRP Bearings - Recommended for contaminating environments.
|Ultimate Compression Strength||70,000 PSI||483 MPa|
|Unit Load Limit PSI||7,000 PSI||48 MPa|
|Temperature Range (Standard Formulation)||±325 °F||±163 °C|
|Coefficient Of thermal Expansion||4.8 x 10-6 in/in/°F||8.69 x 10-6 m/m/°C|
|Thermal Conductivity||1.8-2.3 BTU•in/(hr•ft2•°F)||.26 - .33 W/m*K|
|Water Absorption (2 Hours)||0.12%|
|Water Absorption (24 Hours)||0.16%|
|Maximum Velocity||80 SFM||0.41 m/s|
Any ratings are typical for design purposes. Your design parameters may affect final ratings. Consult with a Polygon sales engineer for guidance. Final testing and approval is the customer's responsibility for their application. This information is derived from our testing and published data. There is no assurance of these properties, or warranty provided that these products are suitable for any particular purpose or operational situation.
Polygon certifies that their product will be free from material defect. Polygon will not accept any liability for loss, damages, or costs from use or misuse of our products.
Specifications are subject to change, and may be affected by our continual process of improvement. Changes may be made without prior announcement.