What Makes This Technology Unique?
The foremost quality of electrodeposited nickel bellows couplings is that they flex to fit misaligned shafts and yet remain torsionally stiff.
“What do a micro-precision silicon wafer cutter, a surgical microscope focusing mechanism, and a target sighting system for an M1A2 Abrams tank all have in common?” Lori Lyons, marketing manager for Servometer & BellowsTech asks. “The answer is that each contains an electrodeposited nickel bellows coupling. Such couplings excel in instrumentation and fractional-horsepower application but are occasionally overlooked because they tend to cost more than other couplings. Couplings are typically rated by their torque capacity, rotational inertia, resistance to the elements, service life, windup and backlash levels, and flexibility. By all these measures, electrodeposited bellows couplings compare favorably to competitive designs.”
The Electrodepositing Process
The electrodepositing process makes this possible because it builds nickel bellows upon a machined aluminum mandrel to a precise thickness and diameter. Dissolving the mandrel leaves a lightweight, convoluted tube. “The bellows is then bonded to aluminum hubs or soft-soldered to stainless-steel hubs. Electrodeposited bellows can be made to just 0.89 mm diameter with walls to 0.008 mm thick, about one-fourth the wall thickness of hydro formed Stainless Steel or bronze bellows.” Lyons explains.
“Also bellows segment radii can be made just three times wall thickness, far smaller than radii produced by mechanical forming. Thinner walls lower rotational inertia which, in turn, cuts drive power losses during intermittent operation. The sharper convolutions, along with the highly ductile nickel material, boost flexibility to minimize side forces between misaligned shafts. A test instrument maker, for example, saved on production costs by connecting a servomotor and tachometer with a nickel bellows coupling rather than a rigid coupling. The bellows coupling paid for itself by eliminating a costly laser alignment procedure,” she concludes.
Other precision positioning applications, such as the silicon wafer cutter, need exceptionally tight mechanisms to keep input and output shafts rotationally aligned. Here, less-costly rigid couplings work because they handle the high torque levels without windup or backlash. However, shafts and bearings may fail prematurely when the encounter angular or parallel misalignment or axial forces.
“Bellow-type couplings, in contrast, tolerate deliberate lateral and axial motion or assembly misalignment yet remain torsionally rigid. Their convoluted bellows transmit torque with negligible windup as they bend, compress, and extend. They can handle over 30 times the angular misalignment and can compress or extend 15 times more than Oldham couplings,” Lyons says.
“Depending on the design, electrodeposited bellows couplings can operate with up to 31° angular or 1.93 mm angular misalignment. Side thrust in most cases is limited to about 4N/.1mm of offset. Good flexibility also helps isolate vibration. Elastic windup is typically less than 10 arc-sec/N-cm, or about one-fourth that of hydro formed bronze or Stainless Steel bellows.”
“This is why a maker of wind-up measurement instruments specified an electroformed nickel bellows coupling to connect a drive to a position encoder. The couplings are also ideal for linking encoders with controls in printing and converting equipment. A high torsional rigidity helps the machines maintain precise registration.”
Understanding the Metrics
Other important metrics include concentricity and service life. The best electrodeposited nickel bellows couplings typically hold concentricity to within 12.7 µ.m. Cyclic speed variation through 360° rotation is effectively zero. When run within recommended limits, the couplings have a near infinite life (108 cycles).
In conclusion, Lyons adds that the designers of the Abrams tank sighting system chose this technology for its high cycle life because they could not tolerate a coupling failure inside the sealed sensor assembly. “Likewise, Global Positioning System satellites use multiple electrodeposited nickel bellows to tune microwave antenna that cannot be repaired in orbit. The seamless, non-porous electrodeposited bellows hold up to the harsh space environment and keep the precision drives they connect contamination free. For many critical applications, electrodeposited nickel bellows couplings can often be the solution to critical applications.”
ServoMeter
www.servometer.com
BellowsTech
www.bellowstech.com