Clunky Knee

[SCzEngrgGroup]

I have a peculiar problem. I've CNC'd the knee on my BP clone, still
using the stock leadscrew and bevel gears. I added two 200# gas
springs to carry some of the weight, and make life easier for the servo
motor. It all works great, except for one odd thing - the knee moves
smoothly and nearly silently when going up. When going down, I get a
periodic "clunk" - about once per rev of the leadscrew. The clunk is
coming from the bevel gears. If I grab the leadscrew with a gloved
hand, providing resistance to turning, the clunking stops. Moving down
requires near zero torque on the driveshaft, so I suspect what's
happening is the leadscrew actually leads the motor in some positions,
which allows the backlash in the bevel gears to open up. Shortly
thereafter, the motor "catches up", and the backlash is taken up with
a "clunk". The question is - what to do about it? Any ideas?

Regards,
Ray L.

[camboy]

sounds like a bit of stiction to me. maybe loosen the gibs a smidgeon to let the knee drop, or bleed some of the pressure from the gas springs if its possible, if not maybe a lighter duty spring would help or remove one to at least prove thats what it is.
j

[SCzEngrgGroup]

sounds like a bit of stiction to me. maybe loosen the gibs a smidgeon to let the knee drop, or bleed some of the pressure from the gas springs if its possible, if not maybe a lighter duty spring would help or remove one to at least prove thats what it is.
j

I just got back from doing exactly that. Removing either gas spring gets rid of the clunk, but it is replaced by a deafening squeal as the knee moves through some positions on the way down. Going up is still smooth and quiet. *All* surfaces on the ways are dripping with oil, so I'm a little baffled by how it can be squealing like that. It appears to be coming from the RH side.

Anyone know how to remove the horizontal shaft? It appears the bevel gear is held on by a key, and a screw into the end of the shaft. If I remove the screw can I then withdraw the shaft out the front? Is there more to it than that?

Regards,
Ray L.

[SCzEngrgGroup]

Problem solved! The gib needed very careful adjustment. It's fairly critical in this case. There's a fine line between too loose, which clunks, and too tight, which over-loads the motor. But, it's now moving smoothly and quietly at full speed both up and down, with both gas springs still in place. I expect stronger gas springs would probably allow me to tighten it up just a hair more, but I don't think that's really necessary.

Regards,
Ray L.