with a twist
Electrodeposited nickel bellows couplers flex to fit misaligned shafts yet remain torsionally stiff.
Paul Hazlitt
Senior Engineer Servometer
Corp.
Cedar Grove,
N.J.
What do a micron-precision silicon wafer cutter, a surgical microscope focusing mechanism, and a target sighting system for an M1A2 Abrams tank all have in common? Each contains an electrodeposited nickel bellows coupler. Such couplers excel in instrumentation and fractional-horsepower applications but are occasionally overlooked because they tend to cost more than other types. Couplers 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 couplers compare favorably to competitive designs.
Making this possible is the electrodepositing process itself. It builds nickel bellows upon a machined aluminum mandrel to a precise thickness and diameter. Dissolving the mandrel leaves a lightweight, convoluted tube. Servometer then bonds the bellows to aluminum hubs or soft-
solders them 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 hydroformed stainless-steel or bronze bellows. And 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 coupler rather than a rigid coupler. The bellows coupler paid for itself by eliminating a costly laser alignment procedure.
Other precision positioning applications, such as the silicon wafer cutter, need exceptionally tight mechanisms to keep input and output shafts rotationally aligned. Here,
Electrodeposited nickel bellows
couplers have less windup and
tolerate greater misalignment than other flexible coupler types.
Electrodeposited nickel bellows couplers handle up to 1.93-mm parallel and 31 angular misalignment.
less-costly rigid couplers work because they handle the high torque levels without windup or backlash. However, shafts and bearings may fail prematurely when they encounter angular or parallel misalignment or axial forces.
Bellows-type couplers, 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 couplers. Electrodeposited bellows couplers, depending on design, can operate with up to 31 angular or 1.93-mm parallel misalignment. Side thrust in most cases is limited to about 9 gm/0.025 mm of offset. Good flexibility also helps isolate vibration. Elastic windup is typically less than 1 arc-sec/N-cm, or about onefourth that of hydroformed bronze or stainless-steel bellows. This is why a maker of windup-measurement instruments specified an electroformed nickel bellows coupler to connect a drive to a position encoder. The couplers are also ideal for linking encoders with controls in printing and converting equipment. A high torsional rigidity helps the machines maintain precise registration.
Other important metrics include concentricity and service life. Servometer typically holds concentricity of electrodeposited bellows couplers to within 12.7 m. Cyclic speed variation through 360 rotation is effectively zero. When run within recommended limits, the couplers have a near-infinite life (108 cycles). Designers of the Abrams tank sighting system chose a bellows coupler for that reason. A failed coupler here would be difficult to replace inside the sealed sensor assembly. Likewise, Global Positioning System satellites use multiple electrodeposited nickel bellows to tune microwave antennas that cannot be repaired on orbit. The seamless, nonporous electrodeposited bellows hold up to the harsh space environment and keep the precision drives they connect to contamination-free.
Electrodeposited nickel bellows couplers from Servometer are rated from 1 to 2,400 N-cm torque. Torque capacity is a function of bellows wall thickness, inside diameter, outside diameter, length, and convolution count. Standard couplers range from 6.2 to 24.03-mm long. Special sizes up to three or four times coupler OD are available.
Some design considerations
Maximum instantaneous torque from a clutch start or brake stop: For a low-inertia motor coupled to a gear reduction drive, multiply motor-starting torque by the reduction ratio. For bellows couplers operating compressed, use 75% of rated torque, max. Extended couplers use normal torque ratings.
Duty cycle: Consider torque spikes, especially for quick reversals or frequent start-stops, such as short stepping moves of an index table. These put added loads on drive couplers.
Axial compression or extension: Electrodeposited nickel bellows compress to 60% of their extended length.
Temperature extremes: Soldered-hub electrodeposited bellows couplers can operate continuously at temperatures from –50 to 126C.
Chemical environment: Nickel bellows will not oxidize in air or alkaline fluids.
Windup: Shorter length or larger diameter couplers lessen windup.