Preventing Binding in Slider track

[RusselME]

I am working on a design that requires a slider that is heavily loaded in track. The slider must be able to translate along the track while loaded. The track is not extremely stiff therefore there will be some deflections.

Does anyone know of any design guidelines for designing such a joint to prevent binding or jamming of the slider in the track.

I would assume these are all variables:
-Slider Length
-Track Relative Deflection
-Waviness
-tolerances between slider and track

Also does slider/track profile matter? i.e. circular vs. dove tail.

If anyone could point me in the right direction, I would really appreciate it.

Thanks

 

[turbo]

Do you have any control over how wide the track can be? Does the slider have friction-reducing aids such as wheels or do you have to reduce sliding friction by use of teflon pads or similar?

If you look at sliding patio doors with tracks, you'll quickly identify some of the problem areas inherent in those designs. If you can describe your application in a bit more detail, you'll probably get some good advice from the ME's on the forum.

 

[Mech_Engineer]

Your best way to go would defineitly be wheels or some sort of roller bearings. If this isn't possible, you need to take a close look at the size tolerances and compare them to expected deflections in the track.

 

[RusselME]

Sorry should of included this information in the first submission.

Slider load is in kips so significantly more than a patio door.

There isn't any room for wheels.
There will be some anti-wear strips. Probably something similar to Teflon.

The track deflections are due to how the track is supported. The track has a relatively low moment of inertia and is support at a few discrete locations along the track. So there is a lot of relative deflections between the supports. We are trying to increase track stiffness to lower deflection but the concern is what deflection is acceptable.

The assumption that there would be some maximum waviness criteria (defl/Len=??) that would prevent binding due to the track deflections changing directions. How would this play into the length of the slider?

I hope this add more information to the problem

Thanks for you help

 

[Mech_Engineer]

The assumption that there would be some maximum waviness criteria (defl/Len=??) that would prevent binding due to the track deflections changing directions. How would this play into the length of the slider?

How much the track can deflect will depend on the geometry of the slider. It should be relatively simple to inspect the slider's geometry and figure how binding would occur given a certain track deformation.

 

[turbo]

Sorry should of included this information in the first submission.

Slider load is in kips so significantly more than a patio door.

There isn't any room for wheels.
There will be some anti-wear strips. Probably something similar to Teflon.

The track deflections are due to how the track is supported. The track has a relatively low moment of inertia and is support at a few discrete locations along the track. So there is a lot of relative deflections between the supports. We are trying to increase track stiffness to lower deflection but the concern is what deflection is acceptable.

The assumption that there would be some maximum waviness criteria (defl/Len=??) that would prevent binding due to the track deflections changing directions. How would this play into the length of the slider?

I hope this add more information to the problem

Thanks for you help

You might need to consider how much bearing surface your load will present to the track. The more surface, the more friction, though the load will be distributed more evenly, but the more surface, the less deflection the track will suffer, so that is a plus, and might let you get by with a track that is not rigid and smooth as you might want.

 

[RusselME]

There will be some space between the slider and the track which will have to be evaluated.

The problem is that it is all analytical up to the test when it matters. So I am trying to find if there are any rules of thumb or design guides out there to supplement the tolerance study.

 

[Danger]

This will probably sound stupid, but I have to ask for the sake of my own understanding. What is the purpose of this thing? I actually do understand the various posts made, but I don't know what is relative and what isn't simply because I don't know the parameters involved. The reference to patio doors, for instance, would not be relevant to something like the suspension system on a snowmobile. With no offense intended toward anyone, could the core requirements of the question be defined a bit better?

 

[RusselME]

Restatement of problem per Danger's request.

The configuration is a track supported at discrete locations along the length. For example the track may be 5ft long and supported only at 1ft intervals. The slider mechanism can be 6in to 1 ft long and is loaded along its length. There is no room for wheels, there is only an anti-wear surface.

Due to the stiffness of the track between supports being unsupported, there will be some relative displacement. The slider has to be able to translate while loaded without binding or jamming in the track.

Does anyone know of any design guidelines for designing such a joint to prevent binding or jamming of the slider in the track.

I would assume these are all variables:
-Slider Length
-Track Relative Deflection
-Waviness
-tolerances between slider and track

Also does slider/track profile matter? i.e. circular vs. dove tail.

If anyone could point me in the right direction, I would really appreciate it.

 

[Mech_Engineer]

This doesn't seem to be as hard as you're making it out to be. The slider will only bind if the gap between the slider and the track is closed by deflection. So make sure you have a large enough gap between the track and the slider to allow some deflection, add some HDPE sliders (or whatever material you want), and go to town.

Profile will depend on what you're trying to achieve and what direction the forces will be in, but I would probably try for either a "/\" profile for good locating, or circular with defined contact points. I have to ask though, why not just use an off-the-shelf solution like linear rails from THK, NSK, etc? You can also build your own out of aluminum extrustion from a company like 80-20 or Item (I would recommend Item over 80-20).

http://www.itemamerica.com/

Example Item slide using aluminum extrusion and side rails:
http://catalog.item.info/medienelemente/PIC_PGR_DYE/PIC_PGR_DYE_Pr-Gruppe-Dynamikelemente_%23SALL_%23AIN_%23V2.jpg

Item has an excellent catalog and a helpful in-house engineering department as well. Just call them up in Akron, tell them what you're trying to do, and they'll send you a catalog and give you some ideas of what you should use in it.

 

[Danger]

Believe it or not, I built one of my favourite devices (a sleeve knife) along the same idea. I used the parts of a windshield wiper blade, plus some springs and parts of a typewriter. Those wipers make awesome sliders, but I suspect that they might not be large enough for your purposes. I would suggest that you just crimp the lower structural tabs of your slider around the track that you intend to use. If you suspect a friction problem, spray the whole thing down with aerosol graphite.

 

[schip666!]

You might examine some typical lathes and mills as the carriages and tables are usually mounted using a sliding dove-tail with an adjustable clearance. Of course they are also much stiffer in each dimension and can stand to have a lot of whaleoil -- well not any more -- lubrication between the surfaces.

 

[Danger]

Oh, yes. Dovetails are both accurate and strong. A little expensive for everyday stuff, but well worth it if they're going to be used a lot.