Shaft couplings.


I had to find a good way to transmit power from the servo motor to the perfectly mounted ballscrew shaft if I was going to get any kind of accuracy from the system.

Bellows style coupling.

Mini Single disc coupling

These two (above and right) are excellent for positioning systems, because they have zero backlash. The only catch is that their torque rating is less than 240 ounce inches. Good for small CNC conversions.

The bellows style are good for positioning systems, they have some backlash. The torsional stiffness is 71,000 inch pounds per radian! Maximum torque 1400 ounce inches. Beyond my price range, but a good coupling..

The MSDC style are great for positioning systems, they have zero backlash. The torsional stiffness is 42,000 inch pounds per radian! Maximum torque 1400 ounce inches. They are shorter than other couplings, and fit into small spaces. Beyond my price range, but a good coupling.

The Oldham style are  great for positioning systems, they have very small backlash! The torsional stiffness is around 7,000 inch pounds per radian,  Maximum torque 640 ounce inches. They are cheaper than other couplings, a good coupling for connecting a motor to a ballscrew.

The curved jaw style are  perfect for positioning systems, they have zero backlash! The torsional stiffness is around 70,000 inch pounds per radian,  Maximum torque transmitted 14,000 ounce inches in bigger couplings. They are more expensive than other couplings, a perfect coupling for connecting a motor to a ballscrew.

The GS Series curved jaw coupling offers zero backlash capability in a 3-piece design. The coupling is provided assembled under pre-stress. These can be used in a variety of

different applications requiring precision and accuracy. The curved jaw series spider features a straight center of the spider tooth, providing higher stiffness due

to coupling pre-stress. The crowning of the ends of the spider legs allows for misalignment, while the curved jaws and solid spider center provide high-speed capability.


The jaws of the hubs and the spider legs are chamfered to provide easy assembly. The curved jaws series coupling design also allows the blind assembly in tight spaces. Raised spider dots on the legs of the spider ensure proper spacing of hubs and spider.


The curved jaw series coupling has spiders available in four different shore hardnesses. Each spider offers benefits for different vibratory, environmental, and torque transmission requirements. The GS Curved Jaw coupling consists of two hubs and one spider.


Simple 3 piece jaw design

Aluminum and steel material hubs

Clamping and locking device hubs available

Four different types of urethane shores to chose from


Typical Applications

Measurement And Control Systems

The torsional stiffness of the GS Series coupling provides zero backlash needed for the accuracy for measurement and control systems. The low torques of

these applications gives the curved jaw series the ability to provide zero backlash due to the elastomer pre-stress.

Servo And Positioning Drives

The curved jaw series provides a zero backlash, flexible connection for servo and positioning drives. An added benefit of the series is its damping capabilities. For

applications that have vibrations at critical speeds, the coupling can provide a zero backlash solution for vibration problems.

Main Spindle Drives

The GS Series coupling is used in main spindle drives for machine tools. Torque spikes and cyclical loading are handled by the series by damping or by

shifting the vibratory frequency range to a non-critical speed range.

Why use a shaft coupling, what do they do?

Shaft couplings are a great! They allow you to connect shafts of one size to another. They allow for misalignment in shafts. See examples of parallel and angular misalignment. No matter how good you are with machining, it is impossible to get a motor and shaft to line up “perfectly” - there is always a small error even if it is hard to measure. A good shaft coupling will take up the error, and prevent motor bearings from being chopped out.


For servo positioning systems it is desirable to have a coupling with either zero, or very small backlash, and a high torsional stiffness. This means there will be no rotational slop, or rotational springy-ness between the motor shaft and ballscrew shaft. If the coupling has a low torsional stiffness, it will flex under rotational load. The more force on the screw, the larger the error due to torque bending the rubber in the shaft coupling. This means that position error of the cutting tool is proportional to cutting force– A situation you want to try and avoid. Example:  A servo motor shaft may have rotated 10 degrees, but the coupling is rotationally springy, it may have only passed on 9 degrees of rotation to the ballscrew, forcing the nut to move 0.125mm, it was intended to move 0.138mm
































Couplings can stop nasty vibration problems because of the damping properties of the rubber.

They prevent a bent or poorly aligned shaft from chopping out the servo motor’s bearings.


They act like a mechanical fuse— if something is wrong and too much force is being transmitted, the coupling is a weak spot which will break and save other components in the system.




There are many types of shaft  coupling on the market and here are a few different with some comments.

Parallel misalignment.

Angular misalignment.