Encoder Products Company rotary encoders are warrantied for three years, and in most cases they exceed that life expectancy by varying degrees. So, what brings an otherwise healthy encoder to an untimely end? More often than not, the cause of shortened product life is mechanically induced. And, in many cases, it's excessive bearing wear. How can it be avoided?
Prolonging Encoder Bearing Life: 6 Factors to Consider
The mechanical life of any encoder is mainly determined by the loading to which the unit's bearings are subjected. Two types of loading must be considered; radial and axial. Radial loading is the perpendicular force applied to the shaft. Axial loading is the parallel force, or force that is applied along the same direction of the shaft. As either the radial or axial loading increases, bearing life is shortened. For this reason, minimal shaft loading or misalignment should always be the goal when installing an encoder. Each of EPC's encoder models have radial and axial shaft load ratings published in the data sheet. For applications requiring heavy shaft loads, consider the Model 725 industrial housing, featured in a previous post. Also, consider a hollow bore or through-bore style encoder with a flexible mount. In this case, the bearings need only support the weight of the encoder.
When considering radial shaft loading, keep in mind that, as the distance between the bearing and the applied force increases, bearing loading increases as a linear function. Much like holding a heavy object with your arms outstretched, the load on the bearings is greater, the further your radial load is from the face of the encoder. With a pulley or sprocket mounted at the end of the shaft, especially when extended shafts are used, the resultant radial loading may actually exceed the rating limit of the encoder bearings. It is always best to place a pulley, sprocket, or wheel as close to the bearing as possible. Also, for measuring wheel applications, a pivot arm with spring tension allows the encoder to move in response to any surface variations, thereby minimizing loading of the encoder shaft.
Shaft Speed The life of a bearing is often defined in total number of revolutions. The accumulation of revolutions goes up dramatically with higher shaft speeds. Another effect of increased shaft speed is increased bearing temperature, which also shortens the life of bearings. Be aware of the impact that wheel, gear or pulley sizing may have on encoder shaft speed.
Temperature Excessive high temperatures can thin out the grease in bearings, causing it to escape. This reduced lubrication will then increase the wear of the bearing. Likewise, low temperatures can adversely impact the performance of bearing lubricant, keeping it from moving freely inside the bearing. For example, an encoder in a cold location that remains static for a long period, and suddenly spools up to 4000 RPMs may experience reduced bearing life. Bearings can handle temperatures normally found in most factory applications, but operating at excessive temperatures will often reduce bearing life. All of EPC's encoders have a recommended operating temperature range specified in the data sheets. These temperature ranges take into consideration the impact of heat and cold on the bearing lubricant as well as other mechanical and electrical factors.
Contamination Foreign matter, whether in liquid or solid form, which infiltrates into the bearings can cause rapid failure. Liquids can dilute or degrade the bearing lubricant, while abrasive materials, such as metal or carbon dust, are most damaging. Shaft seals help protect the bearings from the ingress of any foreign substances. However, some extremely fine particulates can work their way into the shaft-seal interface, and given enough time, ingress into the bearing seal. Consider specifying an IP67 shaft seal, such as is available on the EPC Model 725. It features a unique double lip design that aids in keeping contaminants out. Be sure to keep in mind that the greater the IP rating, the greater the starting torque, which may be critical to some applications.
Proper placement and orientation of the encoder can also greatly reduce the possibility of problems with contaminates. For example, placing an encoder with the shaft facing upward into a water source, will enable water ingress into the bearings given enough time. Gravity will eventually win the battle. Also, shielding the encoder from liquid or solid contaminates may serve to greatly extend its operating life.
Shaft Impact & Vibration Ball bearings are precision components with very critical internal clearances. Disturbing these clearances shortens their life. A very common problem during installation is an impact to the shaft or housing of the encoder, causing the bearing clearances to change. NEVER tap or impact the encoder shaft or housing during installation.
Also, with shaft encoders, it is important to utilize a flexible shaft coupling. Rigid couplings can transmit harmful inputs to the encoder shaft and the bearings such as vibration, shaft end runout, misalignment or other loads. EPC offers a number of industry standard shaft couplings as accessories.
Multiple Effects It's also important to realize that when multiple bearing issues are present, such as high shaft loading at high speeds, bearing life can be significantly reduced. A certain amount of radial load with low temperatures and low rotational speeds might never cause a problem. However, the same amount of radial load with excessive temperatures and high shaft speeds will certainly effect the life expectancy of the bearings.
Conclusion All of the factors mentioned above can be present to one extent or another in industrial encoder applications and can shorten bearing life. As an encoder manufacturer, we take design measures to mitigate the adverse impacts of these factors on encoder bearings. For example, we offer sealed bearings, shaft seals and 80lbs rated bearings. However, before we manufacture the encoder, its even more important that the optimal encoder model and product features are specified. With proper specification, application and installation of rotary encoders, you will maximize the unit's operational life.