Dave’s Corner: Shaft Cushions

Lately we have seen several pump failures due to improperly installed shaft cushions (p/n 770582). We have noticed that the problem occurs when a shaft cushion is installed in a pump that does not require it.

For instance, the shaft cushion is used on all NDP-40 and NDP-50 pumps with rubber diaphragms, including Santoprene® and Hytrel® elastomers. However, the shaft cushions are not to be used on NDP-40 and NDP-50 pumps with PTFE diaphragms or any NDP-80 series pump.

The purpose of the shaft cushion is to prevent the center disk from impacting the air chamber wall when operating at the far right of the performance curve. This isn’t an issue with the NDP-40 and NDP-50 pumps outfitted with PTFE diaphragms because the pumps are destroked and will not travel far enough to impact the chamber. This can, however, happen with the NDP-80 series pumps. A shaft cushion cannot be used because it will interfere with the center disk as it makes contact with the pilot valve.

Lastly, if you have a rebuilt NDP-80 pump and cannot get it to cycle, check to see if it was rebuilt (incorrectly) with shaft cushions. Below is a quick reference guide to follow when determining whether you can install shaft cushions in your pump.

shaft cushion