Figure 1 - Tribological components in miniaturized electrical motors

 

ELECTRICAL BRUSHES

High-speed miniaturized motors rely on brushed and brushless technology. Typical DC motors with continuous running operation with less load and low starting current use precision metal brushes. For high torque applications, carbon brushes are preferred. Motor brushes are always under electrical and mechanical stress, as they transfer current from stationary component to rotary component. It is imminent that brushes wear with time in application. The wear rate of these brushes is dependent on tribological factors such as brush material, collector coating, contact force, sliding speed, environmental condition and lubrication features. Here, wear of brush with respect to commutation is proportional to the spring force applied on brush and sliding speed. Starting current, continuous running condition current and voltage drop across commutation cause electrical wear. When rotating at higher speeds, this sliding electrical contact can cause mechanical losses, unstable electrical contact, joule heating effect and arcing. This leads to surface wear and overall reduction in motor life. To reduce the friction at commutation points, special electrical greases need to be used. A contact model can be seen in Figure 2.

Figure 2 - Motor commutation contact model

 

BEARINGS

 Ball Bearings: Bearings are the important element in miniaturized motors which are tasked to reduce friction between the rotating shaft and stationary flange. Selection of bearings requires detailed understanding of load pattern, system level deflection, bearing type and material, lubricant used, clearances between rolling element, possible misalignments because of loads, temperature behavior and etc.

Hydrodynamic lubricant film formed between rotating and stationary components keeps the wear rate always under check. Understanding lubrication layer physics and opting for correct lubrication based on material compatibility, dew point, viscosity parameters, environment factor and service temperature is required for reliability of bearing and thus performance of motor. Multiple variants of lubrication from oil to grease with different level of thixotropy are used in bearing configurations.

Bush Bearings: Less lubrication-based bush bearings are the preferred choice for low speed, low load type and less service life applications. At higher speeds as well as at higher loads, the bush bearing encounters failures like wear, breakage, cold welding, etc. Some of the sintered bearing holds lubricant which is delivered through capillary action between rotating components. Here, part density, size and amount of porosity and its distribution are critical for life of bush bearing. Sintered bush bearing is a most widely used option due to its low cost and suitability of wide working environment.  For cost sensitive application with less service life expectancy, self-lubricated polymer-based bush bearing is the preferred option.

 

GEARBOXES

Gearboxes are generally used for speed increase and torque reduction and vice versa. Typically, planetary or spur compound gears are two types of gearing arrangement commonly used for miniature motion solutions and mostly these are made up of metal gears for higher torque capacity. For noise sensitive applications, nonlinear materials like polymer with lower stiffness and good damping characteristics are preferred. Other critical factors are time, temperature, humidity, lubricant’s ingredients and glass transition temperature range (for fatigue) of polymer. In Figure 3, you can see a gearbox system which was analyzed for gear failure because of tribological aspects like material strength, temperature, friction, lubricant viscosity, deflection, etc.

 

Figure 3 - Gearbox analytical model

 

Lubrication:  Another challenge for gearboxes is to retain efficient lubricant boundary throughout their life. Most gearboxes fail in the field due to loss of lubrication boundary for various reasons such as lubricant viscosity change, selection of improper lubricant grade, improper lubricant quantity and contamination in lubricants. 

Keeping perspective of long and reliable service, tribological needs must be identified and addressed at early design stages. Motion solutions designed with due consideration of tribological factors during early design stage always outperform many low-cost motion solutions in market.

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