View Full Version : Potential Shop-Bot customer with question

06-01-2006, 06:42 AM
Competitors criticize rack and pinion technology as potential for dust, dirt. Do you run these dry to reduce that potential. Is it a significant problem?

06-01-2006, 07:45 AM
You should grease them, they are positioned as such that they can't really collect any dust. I've not greased or even wiped my rails in over 2 years (i'm a little embarrassed to say...)

The one thing about competitors, is that they are the competition, they need something to tear down on the "other guy's" machines. You need to see a demo live and lay your hands on the machines you are considering. Not a sales person in a sales center, a live shop making money with an end user running the machine. Any company worth dealing with should be able to arrange just such a demo, I know ShopBot can...

06-01-2006, 08:21 AM
Six years here. I may have wiped them off a couple of times and I have greased them lightly several times. No problem with any build up.

06-01-2006, 08:38 AM
If you use a spray on white lithium grease, (available at most large hardware stores) it solves the problem. it dries "hard" over night, and it lubricates pinion gears, and racks without having any tendencies for dust/chips to stick to it's surface. I spray it onto an old toothbrush and then brush it onto all mating surfaces...
A side benefit is that you also get less "track marks" on your clothing as with the heavier grease

06-01-2006, 10:16 AM
I've only had my Bot for a year but I grease the rack about every 6 months. Excess grease gets squeezed out. I have a couple of shirts with "track marks" and I might try Bill's approach with the white lithium. I am surprised how little sawdust gets into the grease.

I was wondering about gear tooth wear before I bought my Bot so I bough a book on gear design. I was amazed to learn that the gear tooth surfaces (if well designed) do not slide with respect to each other on the load side. The contact is a rolling contact up and down the teeth. Take a close look at one and you’ll see the process.

My guesses follow:

1. I think the rolling contact is why they last so long.

2. The grease helps to reduce tooth surface loads where there are imperfections on the teeth.

3. The wear mechanism reduces the effective radius of the arc of each tooth. If the wear is even on all teeth, they continue to function just fine until the wear is severe. The springs on the motors keep the teeth in contact through the wear process (but may cause some sliding contact on the non load side as the teeth wear and this is one of the places where the grease helps).

4. If the wear is extreme, the top of the teeth will touch the gap between the opposing teeth. This is far beyond the time that the rack and pinion should have been replaced and they are a readily replaceable item on the shopbot.

Short version >> They work just fine and take Robs advice -- Visit a user even if you have to travel to do it. Prepare a list of questions and spend some "hands on" time.

06-01-2006, 11:09 AM
Paul, I happened to write these posts (at http://www.mechmate.com/Forum added by Forum admin) (http://www.mechmate.com/Forum/messages/12/840.html) earlier today, you might find them interesting. Spring-loading the gear into the rack causes a lot of slip/wear between the teeth, and grease reduces that wear. Nobody has mentioned problems with racks clogging up.

06-01-2006, 12:05 PM
I have run a PRT, now upgraded to an Alpha, for 5 years now and run it hard! The only thing I do is lube with synthetic EP grease about every 30 days. A small amount each time. I did finally wear out the Z rack last month and replaced it. I have worn out one set of x axis pinion gears and several sets of y axis pinion gears over that time. The pinion gears are easy to replace, it just takes a few minutes. I've also seen what the "Big Boys" face when their ball screws wear out. By the way, ball screws have to be shielded from dust or it gets really ugly. They are less tolerant of dust than rack and pinion.

Overall I'm more than happy with the maintenance curve on my 'bot and yes, in this business, every competitor will run down every one elses machines.


06-01-2006, 12:24 PM
Thanks for all the excellent feedback.

06-01-2006, 12:25 PM

Outstanding post! Anyone interested in the rack and pinion issues should see this. The amimation is the best I've seen showing the rolling contact. Ted might want to include or reference it in the basic pages on the Bots.

Paul Z

I don't know why my "seach since last log-on" didn't catch these posts. I wonder if they were posted while I was in the forum. Have you had anything like that happen to you?

06-01-2006, 12:32 PM

The reason you did not catch these post is because the were on Geralds Mechmate website which is in no way associated with Shopbot..
Just click the link and look at where you have been taken.