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Reducing sideload and friction can improve force sensing stability. The cornerstone of the ORCATM Series Smart Linear Motors is their unique integration of position and force-sensing capabilities within a single direct-drive motor. Detecting output force relies on a highly tuned control system within the stator. However, the force the stator outputs is not always the same as the force applied at the end of the shaft.
The only points of mechanical interaction between the shaft and the stator are two plastic bushings located on either side of the stator. Significant sideloading on the stator can cause bushing friction, which impairs the force transfer between the end of the shaft and the stator. This application note will discuss strategies to reduce sideload and friction to improve force sensing stability. It outlines recommendations for external bearing support, low-friction grease options, and designs that eliminate the need for bushings entirely.
When to Reduce Bushing Friction |
When NOT to Reduce Bushing Friction |
| If force fidelity is a critical feature for the application | If force feedback is not needed or isn't as critical |
| When longer bushing lifetimes are desired |
When the motor's output forces significantly exceed the sideload it encounters |
| Even quieter operation is needed | If motor forces are in line with the shaft |
More Options
1. Using lubrication.
2. Applying bearing support.
3. Entirely removing the bushings.
Lubrication Options for ORCA Motors
The bushings used in ORCA motors are intended to be used without lubrication for most applications. That said, the use of a proper lubricant can greatly reduce the coefficient of friction of the bushings. In their article The Right Lubrication for Plain Plastic Bearings, IGUS recommends “lubrication with mineral oils with metal soaps,” which makes the “coefficient of friction very low.”
Iris Dynamics has found success using Lubriplate No.630-AA Grease.
Bearing Support
When a motor experiences sideload on the shaft, the force applied to the bushings can be significantly amplified, resulting in friction levels that may become unmanageable. Iris recommends attaching a linear rail to the stator and the shaft end to help mitigate sideload on the motor bushings. While any type of linear rail can be used, it is advised to position the rail at least 25 mm away from the shaft to minimize the effects of eddy currents.
Removing the Bushings Entirely
ORCA motors can operate without bushings when the shaft is externally supported. This approach minimizes friction to the lowest possible level.
This process requires the full support of both the shaft and the stator. Typically, the assembly steps for such a system are as follows:
- Assemble the mechanism with the ORCA bushings in place and verify that the system operates fully with the bushings. This ensures proper alignment between the motor and the bushings.
- Remove the shaft and pop out the bushings. Replace the shaft and reassemble the mechanism. Take care with the shaft, as it is highly magnetic.
- Visually inspect the shaft within the bore across its entire range of motion to confirm that the shaft remains centered.
The above image shows the motor's shaft supported from both ends, enabling bushing removal.
The above image illustrates moving the stator to enable bushing removal for low friction.