Whether fitting a thermoplastic polymer bearing or any other bearing, the fit is crucial. Too tight of a fit and the shaft will not turn. Too loose of a fit and the shaft will vibrate soon after the installation of the bearing.
Bearing fitment, whether it is a thermoplastic one or any other, is also important. Improper mounting can cause denting, wear or cracking of the bearing, while the correct fitment can keep bearings operating longer and reduce the chance of bearing failure.
Fitting thermoplastic versus other bearings
Various fitment methods exist, including mechanical mounting with physical force; temperature mounting, which could require heat or cold to be applied to the shaft and/or the bushing; hydraulic mounting, which typically requires a hydraulic piston to impart the necessary mounting force; or oil injection, which requires a pressurized oil film to reduce frictional resistance.
Various methods of securing bushes, especially bronze bushes using mechanical securing devices, are also available.
Some methods of installing other bearings apply equally well to thermoplastic bearings.
Mechanical & hydraulic methods
Many thermoplastics can be fitted using mechanical and hydraulic methods, such as mechanical and hydraulic presses. These methodologies require compression of the outside of the bearing as it is inserted into the housing as well as more compression to the inside of the bearing.
These methodologies ensure an interference fit with the housing and that the running clearance between the inside of the housing and the outside of the bearing is reduced. They also require that the bush is fitted square to the housing and is well-supported, preferably with a mandrel.
In addition, these methodologies require that care is taken by the installer. Many thermoplastics do not respond well to force; A dead blow hammer may, for instance, lead to cracking of the thermoplastic and installation failure.
Temperature tends to play a role in mechanical fitment methodologies. The coefficient of thermal expansion, the measure of how much thermoplastics shrink and expand with temperature, also has an implication for a press fit and for seizure prevention. Thus, for operating temperatures below 0°C (32°F), an additional press fit may be required to ensure that a bush stays in place at lower temperatures. For moderate operating temperatures, from 50°C to 70°C (120°F to 160°F), an extra clearance is required to allow a bush to expand without the danger of shaft seizure. For operating temperatures higher than 70°C (160°F), a bearing may need to be split with a minimum expansion gap, and a press fitting should not be used.
Because of the sensitivity of thermoplastics to temperature, specific temperature methods can be used in conjunction with mechanical methods of fitment.
This is because a thermoplastic is typically formed when plastic pellets are combined and heated according to a given recipe that will produce certain characteristics, such as a given wear resistance, coefficient of friction, swell index or load-carrying capacity in the final raw material. This means that thermoplastics harden at low temperatures and soften at high temperatures, and these temperature ranges are within the parameters for which manufacturers allow.
Because thermoplastics harden and soften with temperature, freeze fitting is particularly useful when installing large bushes, bushes with thin walls or bushes where the length is greater than the shaft diameter. Freeze fitting involves placing the bush in a suitable container with a spacer filling the area in which the shaft will sit. The bush should then be packed with dry ice, with the outside diameter measured to ensure it is less than the housing size prior to removing it from the container.
The use of liquid nitrogen to shrink the bush is another way of ensuring smoother installation. This methodology requires that liquid nitrogen fill the bottom of a container that will eventually be sealed with a lid. Stacked on supports that lie inside the liquid nitrogen, the bush should never come in direct contact with the liquid nitrogen, but it should be sufficiently exposed to the nitrogen gas to cool and shrink. As with the freeze fitting, the outside diameter should be measured prior to removing the bush from the nitrogen-filled container. Safety precautions and personal protective equipment should also be employed.
While freeze fitting and liquid nitrogen can be used in the fitment of certain thermoplastics, temperature methods that include heating excessively should not be employed.
Unsurprisingly, some heating methods that are used in the mounting of other bearings should not be considered. Heating of the bush housing, in particular, is disastrous, since this typically requires heating in excess of the suitable heating range for thermoplastics and will result in melting. What is important to remember is that once many thermoplastics are melted and returned to solid form, they will not have the desired characteristics that the original formula had.
As with brass bushings, mechanical securing methods can be used on thermoplastic bearings instead of an interference fit. For some bushes, for instance, it may be beneficial to use these methods where the operating temperature is above 70°C and a loosening of the press fit may occur as a result of heating and stress relaxation (a process whereby the bearing molds to the shaft housing).
Various types of mechanical securing methods are possible to stop rotation and axial migration. These include the use of:
- Grub or locating screws
- Keeper plates
- Bolts in the case of a flanged bush
- Bonding agents such as epoxy, strong adhesives or sealants, or another metal-on-plastic bonding agent that will operate at a given operating temperature
Installers should always be careful not to place excessive pressure on the bush in the case of screws or keeper plates, and that screws are bonded or secured so they do not vibrate or work themselves loose.
Other than the general tips on mounting bearings, it should be noted that many different thermoplastics on the market may require different fitment methodologies. These thermoplastics might display a variety of properties and characteristics, including their coefficients of friction, likelihood of swelling, compression strength and operating temperatures. It is particularly the swell characteristics and operating temperatures that may affect fitment. These should be carefully considered when discussing fitting.
Thermoplastics that swell
Consider a low-friction, long-life plain bearing material designed for harsh applications, for instance. This thermoplastic is desired for its ability to withstand high temperatures. It is also extremely wear-resistant and has been used in slurry pumps in the harsh mining industry and in pumps in the sugar industry, where hot abrasive content is a familiar occurrence. It also absorbs water and its dimensions can change with time.
As a result, for fitment of this thermoplastic, it would be best to machine it close to the time that it is to be installed or its dimensions may not be suitable for the application at hand. Design criteria may also play a role in ensuring the fit is correct, with thinner walls, grooves or staves in a steel outer casing to ensure the bearing can absorb water prior to installation and that it will fit on installation.
Swell-resistant, internally lubricated thermoplastics
In contrast, consider thermoplastics that do not swell in water and are dimensionally stable so the bearing will be as ready for installation as when it was received or machined.
These polymers are desired for their low coefficients of friction. They are more likely to slide into the housing without the use of a lubricant than various other polymers and the same design concerns that one might have with a thermoplastic that absorbs water do not exist.
Various thermoplastics producers have their own calculation tools for determining what the best measurements are for a bearing given the inputs provided. They also often provide advice on fitting their bearings given these inputs.
The load, sliding speed and the combination of the load and speed will determine whether a thermoplastic is suitable for an application.
In addition, once the suitability of the application for a thermoplastic bearing is determined, the selection of the correct bearing size requires knowledge of the housing size, shaft size, bearing length, maximum and minimum operating temperatures, the total mass supported, the number of bearings supporting the total mass and whether a press fit is required.
Thermoplastic bearings are proving easy to fit when the right know-how is available. They are also often produced as standard ready-to-use items for the pump industry and they can be purchased with the knowledge that these items have been fitted and used for decades in similar applications.
Note: The article describes the methodologies that Vesconite Bearings recommends for the fitment of its bushings. The methodologies outlined have been used for Vesconite, HiTemp 150 and Vesconite Hilube bushings, but all instructions may not apply equally well to other polymers. If there is any uncertainty about how a thermoplastic should be installed, users should contact the polymer manufacturer.
Charlie Simpson is a technical consultant at Vesconite. Vesconite Bearings’ online tool specifically designed for Vesconite and Vesconite Hilube bearings in the pump industry may be a useful starting point for discovering fitment options for the company’s polymers. Visit vesconite.com for more information.