Friction Coefficient and Shaft Surface Roughness
There is a correlation between friction coefficient and shaft surface roughness. Obviously, the friction coefficient is affected by many different factors. If the shaft surface is too rough, wear will increase. Minor surface irregularities cause mutual abrasion between shaft and bearing and form the friction interface.
On the contrary, an excessively smooth surface tends to cause adhesion, meaning the two surfaces stick to each other. Greater force is required to overcome this adhesion, which increases the friction coefficient.
A large difference between static and dynamic friction coefficients increases the tendency of surface adhesion. This easily leads to stick-slip motion and loud sharp noise. After repeated operation cycles, the noise will reduce and the rough shaft surface becomes smoothed. Therefore, stick-slip at low speeds should be avoided in applications. Optimizing shaft surface roughness is essential to maintain stable housing conditions. SAVI recommends shafts with surface roughness Ra0.2~Ra0.8, hardness above HRC35 and hard chrome plating to prevent the bearing from wearing the shaft.
Bearing Wear
Bearing wear resistance is affected by many factors, so it is difficult to accurately predict bearing wear and service life. Multiple tests show the main influencing factors include load, speed, motion type, bearing material and roughness, environmental dust, type of external lubricant, etc.
Generally, bearing wear resistance gradually decreases with rising load, speed and temperature. Wear resistance can be greatly improved if external lubricant is added. If dust enters the bearing, special wear problems will occur. In such cases, plain bearings can effectively extend the service life of machinery and systems. The material’s excellent wear resistance and self-lubricating properties can greatly prolong bearing service life. Since there is no oil or grease on the bearing surface, dirt particles are less likely to attach. Most dust particles will shed without potential damage to the bearing.
However, if hard particles enter the bearing contact surface, the plain bearing can embed these particles into its surface. Operation can remain at the optimal level even with accumulated dust. Nevertheless, it is not only hard particles that can damage shafts and bearings. Soft particles such as fabric scraps or paper debris can also accelerate wear. Under such conditions, the dry-running and dust-resistant performance of plain bearings takes effect, helping enterprises cut costs.
Tolerances and Measurement System
The mounting dimensions and tolerances of SAVI plain bearings depend on material grade and wall thickness. Water absorption and thermal conductivity are also critical properties for each material. Plain bearings with low water absorption can be designed with tighter tolerances. Thicker bearing walls require larger tolerances. For this reason, SAVI plain bearings are available in different tolerance grades. Within these tolerances, SAVI plain bearings installed according to recommended standards can operate within the permitted temperature range and at humidity up to 70%. We can provide application recommendations to support proper bearing use for environments with higher air humidity or underwater working conditions.
Dimension Measurement Method for EP Series Bushings
The plain bearing is pressed into an H7 tolerance housing bore. This interference press-fit ensures bearing positioning, and the inner diameter tolerance of the plain bearing is formed during pressing. A go/no-go gauge is recommended for measuring the bearing inner diameter.
The go end of the go/no-go gauge shall pass through the bearing inner diameter smoothly. The no-go end of the go/no-go gauge shall not pass through.
Bearing Installation
Housing Bore: There are no special requirements for the material of the housing bore matched with EP series plastic bearings. One end of the housing bore must be chamfered at 25° to prevent scratching the bearing outer diameter during pressing. A stepped mandrel shall be used for slow press fitting. Direct striking on the end face is forbidden, as it may cause deformation and dimensional deviation. The inner diameter tolerances of EP series products are achieved after pressing into a standard H7 bore.
Shaft: No special restrictions apply to shaft materials paired with EP series plastic bearings, but SAVI recommends hard chrome-plated shafts. For easier assembly and protection of the bearing’s internal friction surface, the shaft end must have a smooth chamfer transition. SAVI recommends shaft surface roughness Ra0.2 - Ra0.8. An overly smooth shaft may lead to stick-slip or squeaking noise on the friction surface, while an excessively rough shaft will accelerate bearing wear.
Lubrication Recommendations: Although EP series products are designed as self-lubricating bearings, applying an appropriate amount of external lubricant (such as grease) on the friction surface during assembly can shorten the bearing running-in period and extend its service life.
