TechArchive

This article was originally written in the period 1995-2000

Flexible membrane couplers exploit the tensile and flexural properties of membranes (discs) pressed from cold-rolled stainless spring steel. Attached alternately to the driver and driven members, the membranes transmit torque in tension, while flexing readily in bending mode to comply with alignment errors.

These heat-treated membranes can sustain an almost infinite number of flexural (misalignment) cycles. Huco-Flex M couplers are rates for continuous unidirectional and bidirectional rotation up to 15,000 rpm and are typically specified for high resolution measurement devices, high gain velocity or motion control systems, position-critical frictional loads and dynamometers.

Typically, Flex M couplers conduct the transmission through two complementary angles when compensating for radial shaft errors. The greater the distance between the membranes, the larger the radial error than can be accommodated. To cater for the differing levels of radial error, Flex M are made in two standard lengths.

Being pivotal devices, membrane couplers are also available in single-stage form. When linked by an intermediate shaft, these make up into “whirl-free” cardans (floating shafts) that extend the coupler’s reach and accommodate larger offsets. Cardans de-mount into three parts and, in this form, Flex M can be fitted or removed without disturbing pre-aligned shafts.

Misalignment

The maximum radial error should be determined under the worst case tolerance accumulations before selecting a short or long coupling. If correct shaft alignment relies on setting-up by skilled factory personnel, it must be able to be replicated by service engineers. Should the predicted radial error exceed the capacity of standard two-stage couplers, single-stage versions should be used linked by an appropriate intermediate shaft.

Resonance

Excitation frequencies of closed-loop position or velocity control systems can give rise to resonance when the loads are predominantly inertial and the load inertia exceeds that of the motor. In these conditions, the coupler’s torsional stiffness should be such that the natural resonance frequency exceeds 300-600Hz, depending on the dynamics.

Torque

Permissible torque varies according to the nature of the duty. One of the following torque definitions should best approximate to any application.

Peak torque:the maximum load torque sustainable for a minimum of 10<6>static torque reversal cycles at maximum compliance
Application: Periodic operation of valves, switches or other incrementally-operated loads.

Nominal torque:the maximum non-reversing load torque sustainable at maximum compliance.
Application: Unidirectional drives, typically pumps, fans and dynamometers.

Reversing torque:the maximum reversing load torque sustainable at maximum compliance.
Application: Predominantly inertial loads associated with position or velocity control. Typical direct loads: encoders, resolvers and tachogenerators. Typical reflected loads: slide tables, linear positioning systems.

Fixed and floating shafts

Fixed shafts:Couplers with radial flexibility are required when both shafts are conventionally located by two bearings.

Floating shafts:Floating shafts self-align and make a true angle with the adjoining shafts. Radial location is provided by the coupler. Only angular flexibility is required, or desirable, in the coupler.

Huco Engineering
• Tel: 01992 509888
• Fax: 01992 509890
• Contact: Ray Buttifant