Pinion problems?

[rsorensen]

In the process of chasing down a recently developed 'stutter' problem and pulling the pinions off my Shopbot [PRT Alpha?] I discovered worn ridges in my Y pinion and in one of the X pinions [but not the other X pinion]. Here is a picture of the Y-pinion.
3799

Is this wear or is it a manufacture artifact? The X-pinion shows similar but not as severe.

Thanks!

Rick

[Brady Watson]

Rick,
It is most likely normal wear. Your 1:1 PRT Alpha motors have 20 tooth pinions which tend to wear much quicker than 25T on Standard PRTs and 30T on PRS Alpha machines. It's important to make sure that you pinions and racks are clean and wet with grease. Dry gears wear very very quickly.

If you have a good investigative eye, you'll want to check that each pinion/motor shaft is perfectly perpendicular to each gear rack & that each motor mount is void of any twist or play. Under load the mounts can cause incorrect pinion angles and cause premature wear. Strategic placement of shims can often fix this, but most of the time, tightening the motor mount to gantry bolt takes care of this. If all looks well, then it might just be time to replace X,Y pinions. They don't last forever and are one of the Bot's only consumable maintenance items.

-B

[rsorensen]

Brady,

I've re-assembled my machine and in the process of aligning [and checking alignment] the pinions I discovered that my X-axis racks [yes, both X-axis racks] have a significant bow inwards. It is severe enough that out around x=+50 [or thereabouts] I only have 1/2 of the pinions engaged [the outboard portion, the same area where the wear shows up on the pinions]. It appears the racks are put on with double stick tape - is that correct? How do I take these off and correct the alignment?

Rick

[Brady Watson]

Rick,
Before beginning, take a digitial caliper and measure the distance from the edge of your X-rail to the front of the X rack. It should be somewhere in the 3/8 to 1/2" range or so. Make yourself a spacer or two this exact thickness.

You will need to un-bolt both ends of each rack and carefully remove the rack from the rails. If you run a knife down the tape seam and work it, it should come right off. Take care not to tweak the rack by bending it while taking it off. The tape is another matter all together. You will need a fresh sharp utility knife blade or scraper to take off the old tape. Solvents are nearly useless, except maybe MEK - but be aware that MEK will remove the pretty blue powdercoat paint...so I don't recommend it.

You'll also need to clean the racks the same way. You're aiming for surgically clean here...absolutely NO grease residue can be on the back of the rack where the tape goes or the angle iron rail surfaces. When ready, apply 3M VHB (very high bond) to the rack, allowing the roll to do the work. Don't pull on the tape or allow it to wad up. You want a nice consistent thickness.

When it comes time to reinstall the racks, keep the paper on the VHB, and install the end bolts, just enough to help hold the rack in place. Using your spacer(s) you made, space out the rack from the edge of the angle iron, and carefully remove the tape and work the rack into position until you reach the end. TAKE YOUR TIME! They don't call it VHB for nothin'!

After you get both sides up, align your pinions, energize the machine and work it back and forth across the X axis to help seat the tape.

-B

[rsorensen]

Brady,

Thanks. Looks like this isn't going to be a quick fix. Any recommended sources for the tape? A quick look at the 3M site shows multiple VHB tapes - is there a specific one I need or will any of them do?

Rick

[Brady Watson]

Get the 1/2" wide stuff. I bought mine from www.harborsales.net (http://www.harborsales.net) - It was around $80 for the roll.

-B