Bellows With A Twist – Electrodeposited nickel bellows couplings flex to fit misaligned shafts yet remain torsionally stiff.
Electrodeposited nickel bellows couplings flex to fit misaligned shafts yet remain torsionally stiff.
by Paul Hazlitt, Senior Engineer
Servometer® – PMG, LLC
What do a micron-precision silicon wafer cutter, a surgical microscope focusing mechanism, and a target sighting system for an MIA2 Abrams tank all have in common? Each contains an electrodeposited nickel bellows coupling. Such couplings excel in instrumentation and fractional-horsepower applications but are occasionally overlooked because they tend to cost more than other types. 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.
Making this possible is the electrodepositing process itself. Servometer builds its nickel bellows by electrodepositing a nickel layer upon a machined aluminum mandrel to a precise thickness and diameter. Dissolving the mandrel leaves behind a lightweight, convoluted tube, which is the flexible bellows. Servometer then bonds the bellows to the aluminum hubs or soft-solders them to stainless-steel hubs. Electrodeposited bellows can be made as small as 0.5 mm (.020 inch) in diameter and with walls as thin as .008 mm (.0003 inch), smaller in diameter or thinner in wall thickness then comparable mechanically formed hydroformed bellows. The geometry of electro-deposited bellows allows Servometer to fit more convolutions in to a given length then hydroformed bellows, giving them even more dynamic flexibility. Thinner walls lower the rotational inertia which, in turn, cuts drive power losses during intermittent operation. The thinner 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 couplings. The bellows coupling paid for itself by eliminating a costly laser alignment procedure.
Electrodeposited nickel bellows couplings have less windup and tolerate greater misalignment than other flexible coupling types.
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 they encounter angular or parallel misalignment or axial forces.
Bellows-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. Electrodeposited bellows couplings, depending on design, can operate with up to 31° angular or 1.73 mm (.068 inch) parallel misalignment. Side Thrust in most cases is limited to about 4N/.1 mm of offset.
ANGULAR MISALIGNMENT PARALLEL MISALIGNMENT
Electrodeposited nickel bellows couplings handle up to 1.93 mm parallel and 31° angular misalignment.
Good flexibility also helps isolate vibration. Elastic windup is typically less than 10 arc-sec/N-cm, or about one-fourth that of hydroformed bronze or stainless-steel bellows. This is why a maker of windup-measurement instruments specified a Servometer 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.
Other important metrics include concentricity and service life. Servometer typically holds concentricity of electrodeposited bellows couplings to within 12.7Am (.0005 inch). Cyclic speed
variation through 360° rotation is effectively zero. When run within recommended limits, the couplings have a nearinfinite life (108 cycles). Designers of the Abrams tank sighting system chose a Servometer bellows coupling for its high cycle life because they could not tolerate a coupling failure inside the sealed sensor assembly. Likewise, Global Positioning Systems satellites use multiple electrodeposited nickel bellows to tune microwave antennas that cannot be repaired on orbit. The seamless, non-porous electrodeposited bellows hold up to the harsh space environment and keep the precision drives they connect contamination-free.
Electrodeposited nickel bellows couplings from Servometer are rated from 4 to 2,400 N-cm torques.
Torque capacity is a function of bellows wall thickness, inside diameter, outside diameter, length, and
convolution count. Standard couplings range from 18.2 to 62 mm long. Special bore size and custom
designs 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, multi-ply motor-starting torque by the reductions ratio. For bellows couplings operating compressed, use 75% of rated torque, max. Extended couplings 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 couplings.
Axial compression or extension: Electrodeposited nickel bellows compress up to 60% of their extended length.
Temperature extremes: Soldered-hub electrodeposited bellows couplings can operate continuously at temperatures from -50 to126° C.
Chemical environment: Nickel bellows will not oxidize in air or alkaline fluids.
Windup: Shorter length or larger diameter couplings lessen windup.
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