Has anyone used springs to secure carriages to rails?

[PaulRowntree]

Greetings All;
I am considering a new build of a woodworking machine, and would appreciate your thoughts on the solid transport rails based on roller-skatebearings. Regardless of the contact surface (plate, pipe, angles...) there must be a compressive force that constrains the opposing rollers against this contact surface. Most (all?) of the designs that I have seen here on the zone apply this by a static tensioning (such as by squeezing pipes together to constrain the carriage, or with bolts that are tightened once in position).
Has anyone tried this using springs to dynamically pull one set of rollers towards the rail ? Springs would allow the rollers to track minor surface flaws and dimensional errors as well. Lee Springs has extension springs that could easily apply 40-60 lbs of force to constrain a carriage to the rails, which I would think would be adequate for this task.

Any and all thoughts would be very much appreciated.
Cheers!
pr

[ger21]

The weakest point on my machine is the bearings against the pipe. Using threaded rod, I tighten it until the bearings stop rolling smoothly, then back off a little. There's far more tension than 50 lbs, and there's still too much play in the system. Ideally, you need a 100% rigid system with no flex at all, or your cut quality will suffer, or you'll have other issues.

[PaulRowntree]

Originally Posted by ger21 The weakest point on my machine is the bearings against the pipe. Using threaded rod, I tighten it until the bearings stop rolling smoothly, then back off a little. There's far more tension than 50 lbs, and there's still too much play in the system. Ideally, you need a 100% rigid system with no flex at all, or your cut quality will suffer, or you'll have other issues.

Thanks Gerry for your input. Can you estimate how much force is being applied in your system? With 4 springs per carriage, I can imagine a maximum of ~80 lbs of force, and of course, more springs would deliver more force.
Am I over-estimating the need to readjust the roller tension and deal with track flaws?

Cheers!
pr

[Scanfab]

What are the travels you need? Both X and Y axis movement? I found linear bearing on www.ebay.com , a set of 4 rail and 8 bearing blocks that could build an X-Y table with for only $50.. I dont see why your idea wouldnt work but sounds like alot of work and much higher price than getting real linear bearings sourced somewhere other that OEM stores.

[Jkountz]

Most rack and pinion systems use a spring loaded system to keep tension on the bearings. At www.cncrouterparts.com there is a nice video and some pics showing this in detail.

[ger21]

Originally Posted by Jkountz Most rack and pinion systems use a spring loaded system to keep tension on the bearings.

No, they spring load the pinion into the rack, not the bearings.

[ger21]

Originally Posted by PaulRowntree Am I over-estimating the need to readjust the roller tension and deal with track flaws?

Imo, yes. I used EMT conduit, and the bearings are slowly wearing flat spots on them, but they roll very smoothly. I have enough tension to flex the bolts and/or angle that the bearings are mounted to. I have a tall gantry, and it's quite heavy. Rapid direction changes cause the hole thing to shake back and forth. It's barely perceptible, but it's too much.

You won't need to re tension the bearings. You just need a way to get them very tight. If your bearings are riding on pipe at a 45°angle, you need to tension them almost to the point that the bearings are binding.

[PaulRowntree]

Gerry : Thanks for the info. Will consider ...

ScanFab : Well shopped! I have yet to see a deal that good, but it is all a question of timing with eBay. I am hoping for a 48"x24"x6" working volume, and any combination of tracking that works.

Cheers!
pr

[Jkountz]

Originally Posted by ger21 No, they spring load the pinion into the rack, not the bearings.

Which would in turn put pressure on the bearings as well wouldnt you think? How could it not. I agree that the main purpose is to tension the pinion to the rack but that entire assembly is also pulling down on the bearings at the same time I would think. Does this make sense or no??

[ger21]

Originally Posted by Jkountz Which would in turn put pressure on the bearings as well wouldnt you think? How could it not. I agree that the main purpose is to tension the pinion to the rack but that entire assembly is also pulling down on the bearings at the same time I would think. Does this make sense or no??

Yes and no. If you're using DualV bearings, then it's pulling one set tighter and one set looser. But in reality, the bearings should be holding far tighter than the spring pressure on the pinion, so it's not actually adding any holding power.

[Jkountz]

Oh ok see what you're talking about now. I was however thinking of something along the lines of the mechmate setup where there is only the upper v bearings. In other words without the rack an pinion you could essentially lift the gantry right off the table(may be heavy but you get the point) . But once the r and P in engaged and the spring tensioned, you could not. At least not nearly as easy as before since the spring is now applying downward pressure on the pinion and the bearings above.

[ger21]

I didn't realize the Mechmate didn't have bottom bearings. I would think that rapidly plunging would lift the gantry??