Linear bearings on bright bar

[MartZ0r]

First of all, happy new year!

I have not posted on here for a lomg time, my last project was a gym squat rack and i am planning on adding a leg press/hack squat and of corse i have to make it rather than buy it!

Id like to build a high spec one that runs on linear bars rather than the cast poly wheels in a channel. These type cost a bomb!


Now after looking into it, id be looking at 2x 35mm bars with lm35uu bearings. The precision bars cost an arm and a leg. So whats stopping me just using bright mild bar instead? I know theres the precision side of things but its not cnc machine. Maybe the tolerence would be off and the bearings wont slide down it?

The only other factor i can think of is hardness and bearing life? The bearings dont cost the earth and its for home use so my thoughts are it would probably still outlast my use.

Any thoughts on this?

 

[Johnnybravo]

Soft bar becomes serrated very quickly. Movement is then very erratic. At the very least see if you can get some hard chrome bar .

 

[MartZ0r]

Ah right ok. So it would be noticeable then. Hard chome bar is an option i could look. I suppose if it runs close to the cost of precision id be better just getting it. But the cost between bright and precision was a lot so figured it was worth seeing where the issue would be with it.

 

[Blue Chips]

Id like to build a high spec one that runs on linear bars rather than the cast poly wheels in a channel. These type cost a bomb!

If you are still in the design stage, here's a site that lists many different types of linear bearings and related items that might give you some ideas:
https://www.mcmaster.com/linear-bearings/

 

[Brad93]

You want hardened and ground linear shafting to get the most out of linear bearings.

Most gym equipment i've seen is just deleting sleeves on chrome bar.
you can buy the bar from any good hydraulic supplier

 

[Stupoty]

Low friction plastic , you could make plain bearings from it and being plastic it won't wreck the soft metal.

Stu

 

[mike os]

LChrome or stainless tube and delrin sleeves, or fess up and buy proper linear shafting with linear bearings.

Using linear bearings on any old shaft is unlikely to perform well and cost far more in bearings than it should fit the performance you will achieve

 

[MartZ0r]

Thanks for the input guys, going to look at all suggestions regarding other ways, plastic bushingscould be an option to be fair.

This is the cheapest i found the right kind of bar, id need two

https://www.bearingboys.co.uk/Linear-Shaft/35mm-x-1450mm-Long-Linear-Shaft-W35H61450mm-123933-p sorry hope i can post links

 

[Hood]

Do you really need 35mm? What about supported rails? Suppose it depends on the configuration as to whether they are ok or not.
Have a look here for some rail kits, only 25mm max dia though.

supported rail kit

round rail kit

 

[MartZ0r]

I did like the idea of rails, i could drop the size easily. My concern with this was how the weight is loaded and if they are strong enough.

The weights will be loaded below inside the frame, behind the carridge. Rather than on the top of the carridge. This will mean there will be two different angles of stress.

The top carridge will have weight pulling down, and the lower part of the sled, the bearings will actually be pushing away from the rails.

If it were top loaded, i dont think it would effect it. But its more space saving to center load inside.

I mean if i could get away with it, could save me a lot and be very easy to build

Hopefully i explained that properly

 

[m_c]

Low friction plastic , you could make plain bearings from it and being plastic it won't wreck the soft metal.

Stu

Plastic bushings can wear hardened metal quicker than the bushings, let along soft metal.
If any contamination gets impregnated in the plastic, then the plastic acts like a bit sandpaper.

However in this application, contamination shouldn't be too much of an issue, so I'd probably chance some suitably sized stainless, as it'll at least not visually rust.

 

[Stupoty]

Plastic bushings can wear hardened metal quicker than the bushings, let along soft metal.
If any contamination gets impregnated in the plastic, then the plastic acts like a bit sandpaper.

However in this application, contamination shouldn't be too much of an issue, so I'd probably chance some suitably sized stainless, as it'll at least not visually rust.

Yeah it's true the statement was a bit of a blanket statement.

In a high load low speed application like this I would think its ok. also there is the lack of oil/grease/guber on the sliding parts might be nice too.

Stu

 

[Cheesie]

Thought about what you're talking about a few times when I was still lifting, we used to use bright steel for rotating Bridge bearing pins it was never the same diameter most times we had to run a belt/pipe sander over it to get it to fit into the bridge bearings

 

[MartZ0r]

Yea, i thibk im just going to run with using the normal stuff, seems the best idea. I was going to drop the lm35uu's into some tube too, that would cut costs and make assembly easy.

I think when i order the steel lengths i might just ask the stock holder if they have any of this type of bar, might be the only way of getting it any cheaper. Otherwise il just run with it. Saves building it twice and it will last forever. After all these machines to buy are over 2k so cant moan i suppose at a couple hundred on some bar.

Thanks guys for all your input!

 

[Morrisman]

Is the load moving vertically on the bars? Or at an angle? The leg press in my gym moves horizontally but has cables and pulleys connecting to the load. There is also one where the weight is slid up at 45°.

The secret is to have depth to the sliding components, so it's not going to slip sideways a bit and lock up geometrically. Do you have a drawing of the project?

 

[MartZ0r]

I dont if i am honest, just grabbed a few ideas off the net and was going to do a mix and match haha

45 degree plate loaded is the plan, with a lifting foot plate on hinges to transfer it from a hack squat to a leg press. I was going to throw some ideas together once i knew the exactly slide mechanism to go for

 

[Morrisman]

I dont if i am honest, just grabbed a few ideas off the net and was going to do a mix and match haha

45 degree plate loaded is the plan, with a lifting foot plate on hinges to transfer it from a hack squat to a leg press. I was going to throw some ideas together once i knew the exactly slide mechanism to go for

You'll need two bushes on each upright, as far apart as you can get them, to maintain geometry. The further apart the better, it will slide much more easily. Nylon inserts running on mild steel tubes will last 10,000 heavy squats.

https://fitnessprogramer.com/wp-content/uploads/2021/02/Sled-Hack-Squat.gif

 

[bricol]

Igus bushing website has a lot of info on plastics and shafts. On-line shop too.

 

[MartZ0r]

I have considered the plastic route, and out the two i think i would still sway to linear.

There is another option, (always worth re-thinking a little) i have seen older style leg presses use, and thats simple PFC channel, with some round stiched to the inner and outer flange, and use some steel or nylon U rollers. It would cost a heck less.

Only reason i thought of it is, my old local gym had one that was simply running in 125mm PFC with 100mm cast wheels, wasnt exactly quiet but i cant remember it feeling hard to use.


I can see how commercial gyms would be more attracted to linear type as its more modern.


Pfc and rollers will add more resistance, but the more i think of it, is that really an issue as long as it dont bind and is consistent! 350-400k id be loading, but obviously best to over engineer.


It would need 2 u rollers on the inner flange's both sides, and one either side along the back. Could run on 15 or 20mm bright. Would depend on the groove of the wheels really