Need To Access Pinion Gear on 2006 PRT Z Axis

[gerryv]

I have a late model 2006 PRT Alpha. My Z has about 1/16" vertical play which I can cause with little effort. Based on others posts I've read, I'm guessing that it may be a loose pinion gear. None of the other pinions have any appreciable wear so, again I'm guessing, it seems that pinion wear is less likely. Either way, I need to get in there, assuming this is a good place to start my troubleshooting.

Where I need advise is how best to access the Z pinion as I'm not seeing a simple way. Do I maybe need to run the gantry out past the stops to a point where I can drop the tracked router mounting plate (not sure what it's called) far enough down past the table to get at the pinion inside the Z assembly and, if yes, is there some sort of stop that must be defeated to then drop it that far?

Thanks kindly, Gerry

[bleeth]

Unbolt the motor. Then change the pinion and bolt the motor back up. Make very sure that the 2 allen nuts are on the flat parts of the shaft and well tightened. This could be your problem now rather than a worn pinion.

[gerryv]

Hi Dave, Thanks for the input. The problem I'm running into is that the heads of the machine bolts used to mount the stepper motor are inside the Z car assembly so I need to get inside to access them, or so it seems (scratching head). The machine screws are threaded into the Z car housing so no nuts.

[ken_rychlik]

With the power off, you can move the z plate up and down until the holes that line up with the bolts can be gotten to. Reach through the holes with an allen wrench.

[bleeth]

I'll add that you should have your control box off and unplug the motor before you move the z (or any axis) by hand.

[gerryv]

Kenneth, Thanks, that helps too. Unfortunately, only one of the machine screw heads lines up with a hole - the motor is mounted like a baseball diamond if that makes any sense. The machine screws are at the 3, 6 and 12 o'clock positions which means there is only one hole anywhere near in line. Only three of the mounting holes on the stepper are actually used, which will make the job a bit easier. I'm thinking that I'll need to get a set of those Allen keys with the ball end, which may allow me to access the 12 and 3 o'clock ones. There's no way to get near the 6 one.

This might be enough though. I've been peering down from the top into the assembly and it is clear that the play is actually backlash between the rack and pinion. There is no movement of the pinion, even under some pressure, when the stepper is powered up but the backlash remains. There is no "eyeball" visible wear on the pinion gear.

This leads me to believe that I might be able to resolve this either (1) by loosening two of the three machine screws that I hope to reach with the offset Allen keys and then using a long wooden wedge from the open top of the assembly to force the motor/pinion further into the rack and retighten, or (2) removing the rack and shimming it closer to the pinion. The first would be easier because I could do it without having to get inside the Z assembly somehow, which is my basic dilemma.

Does this make any sense, especially in terms of keeping it simple?

[ken_rychlik]

What about drilling a hole to where you can access the other bolts?

Mine was a geared motor mounted square to the frame, so what you are saying does make sense. The direct drive motor is mounted differently.

[gerryv]

I think that's what we call thinking outside of the box eh. I like it a lot. It'll have have no negative effect on the strength of the member so a new hole it will be.

Thanks everyone; another thumbs up for this forum :D

[ed_lang]

Remove the sticker on the z and you will find a hole.

I have removed a direct drive motor and installed a gear reduction motor back into it's place. It can be done. I also removed the springs and unplugged the motor cable from the driver. You will be able to lower the z and reach the Allen bolts. While the motor is loose you will be able to adjust backlash.

I did not need to drill holes.

[Brady Watson]

There are several things you need to keep in mind when adjusting pinion lash on a PRT. The v-rollers set the lash/mesh with the rack mounted on the t-rail and the pinion. The preferred method of replacing the Z pinion and setting the lash is as follows:

1) Disconnect the springs on the Z axis from the bottom spring perch.
2) Remove spring perch.
3) Remove stop bolt located on Z tower plate (near motor)
4) Lift the entire t-rail with spindle up and out of the tower.
5) Remove motor, or if you have enough room, remove pinion from motor while it is mounted.
6) Set pinion on shaft and slide t-rail back into place. Note how pinion meshes with rack, front to back on rack. Adjust and tighten as necessary.
7) With t-rail in tower, slide Z up & down - then turn on control box. Listen & feel for play in the t-rail.
8) If there is excessive play in t-rail, then carefully and methodically adjust the v-rollers on the Z tower to remove this slop - All the while observing and noting that anything you do to one bank of rollers, you must do EXACTLY the same to the other side/bank of rollers - AND that adjustments affect both pinion to rack lash AND t-rail to roller tension.

Take your time making adjustments and it will be a very long time before you have to mess with it again. Do a large area clear/surface your table to verify that the adjustments you made still keep the router perpendicular to the table surface. You can read the ridges left on the table to see if the router is sitting at a slight angle - adjust as needed.

Also - DO NOT OVER TIGHTEN V-ROLLERS AGAINST T-RAIL - YOU WILL SPREAD THE TOWER APART OR OVERLOAD THE STEPPER BEARINGS AND WEAR OUT YOUR T-RAIL!

-B

[gerryv]

Thanks much Ed, It's so easy to overlook these things sometimes. I'm not in the shop right now but will be looking for a sticker for sure. That's an interesting comment about adding the geared stepper on your Z and good to know that it can be done. I'm kind of having fun seeing just how much I can do to squeeze the most precision out of this machine without getting silly with how much I spend. All of my steppers are direct drive so I may also be going the belt-drive route for the other axes but I can see that a geared head on the Z would be a more practical approach.

[gerryv]

Very enlightening Brady, I appreciate the detail. In fact, it makes me realize that I may have been the cause of the backlash. Last week I discovered that my Z was badly out of tram when I surfaced the table for the first time. Once I could not bump the whole Z assembly any further towards perpendicular on the Y axis (with mounting bolts loosened) I tinkered with the V roller sets as you suggest to bring it the rest of the way. I can see now how I may have backed the pinion gear off the rail resulting in backlash.

With what I've been learning from this thread, I'm feeling more confident that I should be able to go back in and resolve this by first working on getting the Z car perpendicular with the eccentric V rollers set back in their "middle" position. Once that's done, I'll then go to your procedure above and use the V rollers to mesh the pinion and rack properly. Thanks for the insight on how to accomplish that.

I'll take extra care not to over tighten the V-rollers against the T-rail. In my case, I have 8 rather than 4 of them on the Z as I believe this was one of the last PRT's built before the switch to the PRS series and Shopbot was continuing to work on rigidity.

[knight_toolworks]

It could be that the motor slipped a bit too. I have had that issue. Also someone left a washer out on the top of the rack and that caused me slop.

[gerryv]

Mission accomplished; thanks much for the help everyone. Now what can I do next...