bearing rail straightness

[theprof291166]

Dear Fellows
I am a rookie here so please bare with me:
I am trying to reset two plain bearing rails, each rail is around 50 inches long, 1 inch deep and a little less wide. They have triangular sides for the plain bearing pads to track. The rails have around 28 hex bolts I think holding it to the supporting beam. A laser scanning motor is suspended on a cross scan assembly (like a train) supported with two bearing pads on one side and a third single on the other, which is also spring loaded. (This is a Lightjet 5000 laser printer)
The problem I have is getting the rails to be exactly parallel with each other. I managed to get them within 0.04 mm but it doesn't seem to be enough, as the cross scanner projects a laser dot about 25 inches, which of course magnifies any error in the rails. The rails themselves are slightly bowed, and I had to strap and push the rails into a straight line. I used my digital callipers to compare the distance between the rails at regular intervals. It isn't right yet. Is it normal for the rails to be 'bent' (by about 1mm)? How can I get these rails straight? Thanks a lot.

 

[MikeHalloran]

I'm a little fuzzy on the geometry, but it smells like the rail under the two-pad slider is the only one that has to be straight. The other one just has to be flat and coplanar with the first.

If you can stop the scanning motor, and reflect the beam out of the machine, e.g. onto a very far wall, you can use the laser to indicate trajectory errors while you screw with the rails.

Okay, I have to ask; why were the rails disturbed in the first place?




Mike Halloran
Pembroke Pines, FL, USA

 

[theprof291166]

Dear Mike
yes I am somewhat regretting i did take them out. However I took them out to polish the rails as I was getting rather bad jitter effects on my prints. It has improved them that is for sure. I figured what you did, that the two pad rail was the one that defined the straightness. However the other rail I got as straight as possible because it was 'pinching' which could be felt as stiffer at the ends and looser in the middle. Thanks for the reply.

 

[SincoTC]

I agree with Mike, especially when he says that just the two pad rail has to be straight and the other just FLAT and coplanar with the first. However, if I read your post correctly, you're saying they BOTH have the same triangular shape, this is bad news as it's not a good kinematic layout and would require very demanding setup. Maybe the spring loading you mention gives it some compliance, I'm not familiar with the machine (thankfully)!

I would say it's pretty normal for a bearing rail of that section and length to be bowed from the residual stresses of hardening and grinding, it's not an inherantly stiff section and any decent design of machine would have an accurately machined abutment (usually on the inside), to which the rail would be pushed or clamped while the fasteners are fitted and torqued up (from the middle outwards).

Good luck

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2009x32 SP1.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.11.7751

 

[theprof291166]

I have uploaded a picture of the rail arrangement inside the drum. The rails are at the top.

 

[SincoTC]

Very interesting picture and not a bit like I had imagined!

Do you have the manufacturer's manual or have you spoken to their engineers? I expect they set up the rails before they mount the drum and probably have it the other way up too.

Surely there has to be an abutment or some reference face on the supporting beam to get the first rail in the right place. Using digital calipers to set them up is not going to be very easy and if you have to work that way up inside the drum I don't envy you the job. I would always try and use a Dial Indicator with some kind of groove or pins in its base so that you can slide it accurately along one rail with the stylus running against the other rail, you'll very quickly see where it's pinching. Although, in the absence of a location for this first rail, you'd need to establish that it is straight and aligned with the machine axis by sliding the dial gauge on it with the stylus on some convenient linear machined edge before you can set the second rail. I have done this many times, but only with recirculating linear bearing with the gauge mounted on one of the bearing's carriages. With plain bearings, you'll have to make sure that the the gauge block is bearing against the relevant faces, ditto if you're using calipers, are you measuring across the actual bearing faces or some convenient edge that may not be exactly colinear?

As before, good luck!

Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK

SW2007x64 SP3.0 Pentium P4 3.6Ghz, 4Gb Ram ATI FireGL V7100 Driver: 8.323.0.0
SW2009x32 SP1.0 Pentium P4 3.6Ghz, 2Gb Ram NVIDIA Quadro FX 500 Driver: 6.14.11.7751

 

[tygerdawg]

If this is a commercial piece of equipment, then it was probably assembled at the factory with a precision jig to get the required parallelism. That's the best approach for precision.

Parallel bearing rails are set in industrial equipment via a variety of methods to give the required accuracy. Perhaps you can download bearing manuals from INA Bearings, SKF, or others. They usually have methods described of how to mount their bearing rails.

Generally I have always had a machined Mounting Plate that provided datum surfaces for the bottom+outboard locating surface of the rail-1 and bottom locating surface edge of rail-2. Mounting Plate machining was done in a single setup to achieve the required squareness, perpendicularity, and parallelism of the datum mounting surfaces. "Sideways" screws forced the rail-1 to the outboard datum edge, and then it was secured to the bottom datum edge with "vertical" screws. Then the rail-2 was allowed to float as I iteratively moved the carrier rail to the extremes. Then finally fixed rail-2 with "vertical" screws. I hope my description is clear enough. But certainly you can get the idea from the catalogs or engineering manuals for linear rails.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[theprof291166]

I set this up relative to the drum as that is the only reference there is. There may be a datum but I can't find it so it must have been jigged at the factory. Unfortunately this is an obsolete machine, the factory long since gone.
There is a sharp edge to the drum and it wasn't that hard to get a calliper reading from the edge to a convenient edge of the machined part of the rail.
I don't want to remove the beam as this is the only other reference I have as trying to reset that will only add to my variable woes.
Thanks for the advice so far.

 

[unclesyd]

Is there a laser alignment company in your vicinity. You need one that specializes in aligning machine tools. If the set is amendable to check this way they should be able to set one or both rails. If you can get one rail set you could use a gauge jig to set the other.
The company we used had a Hamar Laser setup. If you use a target like the Hamar does you could use your laser as posted by MikeHalloran


Is there room to use a long straight edge for the datum? It wouldn't have to be set against the rail but could be offset ?

Are you sure there is no witness marks on are where the rails mount? They would probably be quite small.

How long are the rails?

 

[theprof291166]

Rails are about 50 inches. Interesting about the laser alignment. I can look into that.
What are witness marks?

 

[unclesyd]

Witness marks are small marks put on components to make aligning or orientating them easier. They are generally straight lines that need to line up. They are normally not very long and are inscribed so as not to cause any disruption of material.

I hope you can find a technician with the laser that likes a challenge. Our first pass at using an outside company we were quite fortunate that we got one who wouldn't say it can't be done. Like as posted if you could use your laser with a proper target it would be great.