Couplings meet rigorous requirements of precision positioning systems

May 1, 2000
Light-duty positioning applications often call for shaft couplings that ensure accuracy in the motion control feedback loop and accommodate shaft misalignment. Here’s one type that fills the bill

Precision motion control applications require mechanically tight systems, especially between motors, driven loads, and feedback devices, to ensure accurate positioning. Such systems often require shaft couplings that are both torsionally rigid, to accurately transmit rotational position, and laterally flexible, to accommodate shaft misalignment.

Light industrial precision applications, for example, often include bellows-type flexible couplings to connect powertransmitting devices, such as step motors or servomotors; or positioning mechanisms, such as ball screws or lead screws; to motion control feedback devices.

The feedback devices are usually of two types: position sensors, such as optical encoders, resolvers, and potentiometers, and velocity sensors, such as tachometers. In a typical system, a motion controller translates the sensor feedback into instructions for the motor.

Coupling details

Bellows couplings typically provide torque capacities from 2 oz-in. to 20 lb-ft for shaft sizes ranging from 0.09 to 1 in. Because of their flexibility, the low-torque versions handle angular misalignment up to 31 deg or parallel misalignment (shafts parallel but offset) up to 0.076 in. In general, as misalignment capability goes up, torque capacity goes down.

These couplings generally exhibit both low windup, due to radial rigidity, and low side thrust, because of lateral flexibility. Other coupling types have either low windup or low side thrust, but not both.

The ability of a bellows coupling to absorb axial movement makes it valuable in applications where you need to compress the coupling to install it or where a shaft translates axially. However, maximum rated values of angular misalignment, parallel misalignment, and axial movement are mutually exclusive. If combinations are needed, consult the manufacturer.

Bellows couplings are generally not suitable for applications requiring over 20 lb-ft torque or less than 0.17 arcsec/ oz-in. windup. Also, high-volume, low-accuracy applications may be able to use other types of couplings that cost less.

Many hub styles are available, but the setscrew hub and split hub with clamping collar are most popular. A split hub is convenient when you need to install the coupling and then adjust the shaft position.

In general, the procedure for selecting a bellows coupling is to determine the maximum torque, parallel or angular misalignment, and allowable windup, then compare these values with the manufacturer’s catalog ratings. These catalogs usually explain how to calculate predicted coupling life.

Positioning applications

Here are some examples of positioning applications that use bellows couplings.

NC lathe. A major manufacturer of NC lathes uses a coupling to connect a lead screw to an encoder, Figure 1. In each machine, three of these encoders control motion of the tool fixturing in the X, Y, and Z axes to achieve machined part tolerances of less than ±0.001 in.

In this low-speed, low-torque application, the coupling accurately transmits lead screw position to the encoder. Any backlash would produce an error in the encoder readout, making it impossible to achieve the tight machining tolerances required. A bellows coupling was chosen because it has a onepiece flexing element with no backlash and only 5 arc-sec/oz-in. windup. This type of coupling also limits side thrust to less than 5 oz- /0.001 in. offset, thereby minimizing premature bearing wear.

Motor test stand. Manual testing of subfractional dc motors by a southern manufacturer was a laborious task, requiring 3 days to test 10 motors. Using a single dynamometer test stand, engineers measured data at five load levels for each motor.

To speed up this testing, they developed a process for mounting the motors in the test rig more quickly and generating torque-speed and torque-current curves with a computer.

The coupling, Figure 2, compensates for minor runout of the mounting fixture, thus eliminating the need for accurate alignment between the motor and dynamometer. The resultant faster mounting, in combination with computerized curve generation, reduced the time to test 10 motors (and plot their performanc curves) from 3 days to a mere 30 min. In addition, the coupling provides zero speed variation and transmits exact motor torque to the dynamometer.

Welding fixture. A manufacturer of small welded components uses a bellows coupling in the chamber of an electron beam welder, Figure 3. The coupling connects an externally driven shaft to a fixture that holds the components to be welded. A motor and belt drive outside of the chamber rotates the shaft slowly to permit 360-deg welding around the part.

Because the components are joined at varying angles — up to 45 deg from the shaft centerline — the coupling needs to accommodate the required angle within a short distance while turning. The selected coupling has an outside diameter of 0.75 in. and an active bellows length of 1.25 in.

The coupling must be made of metal because the welding chamber is evacuated and any rubber or plastic could outgas, contaminating the weld joint. The rubber that seals the drive shaft at the chamber wall is formulated to minimize outgassing.

Stamping press. A manufacturer of stamping presses uses a bellows-type coupling to ensure the accuracy of die position adjustments during press operation. Parts being manufactured are measured with an optical device linked to a computer. When the computer determines that parts are approaching their tolerance limit, it sends the required number of pulses to a step motor that is connected to an adjusting mechanism by the coupling. The motor then rotates, adjusting the die position.

The coupling is flexible enough to handle misalignment and absorb vibration of a 20-ton press while accurately transmitting step motor rotation to the adjusting mechanism. Misalignment is minimal, less than 0.010 in.

Military application. In this case, a major defense contractor needed a coupling to transmit torque between shafts offset by a 30-deg angle. The coupling also needed to guide an array of low-voltage control wires and act as a conduit for oil that flows through connected hollow shafts.

To achieve these unique working parameters, engineers designed a three-ply bellows consisting of separate plies of nickel walls, one inside the other. This configuration has three times the pressure capacity of one ply, but approximately the same flexibility. The pressure of oil inside the coupling pushes against all three plies, but, because the plies are slightly separated from each other, they still flex independently.

Tom Guarino is an application engineer at Servometer Corp., Cedar Grove, N.J.

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