Contraction of metallic parts and hardening of non-metallic parts are the two primary factors that can render a step motor non-functional in low temperatures. In the instance of contraction, if motor components with critical dimensions contract at different rates, a locked-up motor may result. The resultant stress can crack metal parts made brittle by super-cooling. To combat these effects, special alloys must be selected for a low temperature motor, and all metal components must have comparable coefficients of thermal expansion.
Both cable insulation and bearing grease are susceptible to hardening at cryogenic temperatures. Dry lubrication may be required and insulation polymers must be carefully selected to retain molecular integrity through low temperature cycling.
The following case history, which combined two separate hostile conditions in the application environment, represents a typical challenge faced by the design engineer working with a cryogenic application.
Cryogenic Motor Case Study
In a recent cryogenic application, engineers at Arnold Air Force Base were were assigned to identify and qualify motors suitable for use in a vacuum chamber at cryogenic temperatures. The specification called for small motors operating in a vacuum of 10-7 Torr at liquid hydrogen temperatures (24o Kelvin). Angular position feedback was a requirement to accommodate closed loop velocity and position controls.
Two major design problems are inherent in this type of application. The first involves the outgassing of lubrication and insulation materials at low pressure. The second problem has to do with the behaviour of motor materials at very low temperatures. The stress of large temperature changes, low temperature brittleness, and varying contraction rates of dissimilar materials work to degrade the structural integrity of motors manufactured from conventional materials.
A 57 mm diameter step motor was combined with a feedback resolver to meet the unique requirements of the application. Both devices were housed in an exotic nickel chromium steel alloy frame, selected for thermal stress resistance and dimensional stability. Resolver technology was selected for the feedback system due to the similarity of resolver components with those of the motor.
To reduce outgassing at low temperature, insulation materials were made of selected polymers. Magnet and lead wire materials were carefully specified to avoid outgassing or fracture. Bonding agents normally used to build the motors were replaced with adhesives having a coefficient of thermal expansion close to that of adjacent steel components.
Each of the metal components of the motor was examined in detail. AlNiCo (Aluminum Nickel Cobalt) magnets were selected in favor of rare earth combinations, since AlNiCo retains magnetic properties better at low temperatures. Stainless steel ball bearings, lubricated with dry film, were used for the same reason. All machined metal parts were stress-relieved. The result was a design that could operate at cryogenic temperatures and within the confines of a vacuum chamber without vaporization of motor materials.
Recent Low Temperature Applications:
- Satellite controls
- Antenna controls
- Observatory instrumentation
- Liquid oxygen pumping
- Superconductor research
- Plasma Processing
- Frozen food handling
- Paper mills
- Steel forming
- Metal coating