Need an engineers help!

[pattieG]

I'm building a piece of gym equipment and this piece will have a carriage that carries weight up and down a rail that is mounted at 45 degrees. I have got some heavy duty nylon wheels that I will be mounting between two pieces of steel. I was thinking of having a thrust bearing either side of each wheel to prevent it binding when tightened but they are £10+ each and I will need 12qty. I then thought that I could use some PTFE washers and save £100+.

My question is can I use the PTFE washers to achieve the same effect as a thrust bearing? The load of the carriage will be transferred vertically into the rail and the only horizontal force on the PTFE washers will be from the tightening of the bolt.

If this thread is in the wrong section then please say and I will ask a moderator to move it to it's proper place.

[dirtbiker]

Don't see that you'd have any issues with PTFE washers in that application. Don't tighten the wheel nuts up too much though.

[Frik]

Fag packet sketch required I think.

[Leptons]

Have you seen the price of PTFE recently? Roller bearings can take a little side load anyway, at least enough to keep themselves on your track arrangement I'd have thought. Are you sure you need thrust bearings?

As above, fag packet sketch required.

[pattieG]

Here is the carriage assembly, the steel plate to hold the wheel on the nearest side has been removed for clarity.



Here is what I am building

[Snake the Sniper]

Rather than use 12 nylon wheels and associated bits, could you just use 4 bigger steel bearings and run them direct onto the 45 degree rail? Roller skate bearings should do it, unless going over 150 kgs or so I'd guess and won't be much.

[pattieG]

I want it to be able to handle a lot of weight and I already have the wheels.

[Snake the Sniper]

Just how much weight are we talking? Just because you have the wheels doesn't mean they're the best/cheap[est option in the long run.

[Frik]

That isn't the problem in hand though.

How were you planning on mounting these PTFE washers? In pairs? I can't really see them being as effective as a decent thrust washer setup in this scenario. Of course, ideally you need to do some number crunching to be sure.

[pattieG]

Snake the Sniper said:

Just how much weight are we talking? Just because you have the wheels doesn't mean they're the best/cheap[est option in the long run.

1200kg is the criteria. I have six wheels rated at 200kg each. In reality I will probably never load more than 50% of max weight onto the press but I want it to be able to safely handle that 50%.

Frik said:

That isn't the problem in hand though.

How were you planning on mounting these PTFE washers? In pairs? I can't really see them being as effective as a decent thrust washer setup in this scenario. Of course, ideally you need to do some number crunching to be sure.

I was going to mount them in pairs on either side of the wheel. I would love to use thrust washers but they are out of my price range.

Unless you have 12qty M8 thrust washers lying around?

EDIT: Just ordered 20qty thrust bearings from china for £12, lets just hope they're not ste!

Edited by pattieG on Thursday 16th August 12:59

[TROOPER88]

That looks like an incline leg press.
If this is the case you will need to add a locking mechanism somehow. Imagine you can not complete a rep you need to be able to lock it somehow so enable you to get out of the machine.

I bet you know this already. Good luck

[Snake the Sniper]

TROOPER88 said:

That looks like an incline leg press.
If this is the case you will need to add a locking mechanism somehow. Imagine you can not complete a rep you need to be able to lock it somehow so enable you to get out of the machine.

I bet you know this already. Good luck

That was my fist thought too, until he said it's designed for 1200Kgs.

[Pastie Bloater]

pattieG said:

1200kg is the criteria. I have six wheels rated at 200kg each. In reality I will probably never load more than 50% of max weight onto the press but I want it to be able to safely handle that 50%.

No offence to you but this does worry me. For the level of weight you are talking (1200 kg), if it not a typo, you need to carefully design this equipment to stop it killing someone. You need to take a look at EN 957 (ISO 20957) which is a legal requirement. Is this equipment for home (domestic) or gym (commercial) use? If I assume the former you need to account for a dynamic coefficient of 1.5 and a safety coefficient of 2.5. This is why 1200 kg load and 6 wheels rated at 200 kg each worries me. As a further example your weight bar needs to be able to support between 2-4 times the rated capacity at its mid-point. There are further tests for cyclical loading, limits on access to squeeze points, intrinsic and extrinsic loading etc.

Are the bolts going all the way through the carrier? Do the nylon wheels have integral roller bearings? You appear to have some very small rollers inside the carriage, depending on what these are you may be able to use the PTFE washers as suggested.

What calcs have you got so far?

Edited by Pastie Bloater on Sunday 19th August 16:39

[pattieG]

TROOPER88 said:

That looks like an incline leg press.
If this is the case you will need to add a locking mechanism somehow. Imagine you can not complete a rep you need to be able to lock it somehow so enable you to get out of the machine.

I bet you know this already. Good luck

I have a spotter arrangement in mind, it's just not on the drawing.

Snake the Sniper said:

That was my fist thought too, until he said it's designed for 1200Kgs.

1200kgs is complete overkill, I don't even have a third of that weight in my gym!

Pastie Bloater said:

No offence to you but this does worry me. For the level of weight you are talking (1200 kg), if it not a typo, you need to carefully design this equipment to stop it killing someone. You need to take a look at EN 957 (ISO 20957) which is a legal requirement. Is this equipment for home (domestic) or gym (commercial) use? If I assume the former you need to account for a dynamic coefficient of 1.5 and a safety coefficient of 2.5. This is why 1200 kg load and 6 wheels rated at 200 kg each worries me. As a further example your weight bar needs to be able to support between 2-4 times the rated capacity at its mid-point. There are further tests for cyclical loading, limits on access to squeeze points, intrinsic and extrinsic loading etc.

Are the bolts going all the way through the carrier? Do the nylon wheels have integral roller bearings? You appear to have some very small rollers inside the carriage, depending on what these are you may be able to use the PTFE washers as suggested.

What calcs have you got so far?

Edited by Pastie Bloater on Sunday 19th August 16:39

Well it is purely for my use only and will not be sold to or used by members of the public so I don't need to conform to any standards whatsoever.
As for squeeze points there is very little and there will be guards to prevent myself crushing anything really valuable.

The nylon wheels do have integrated roller bearings and they are 80mm in diameter. They look small when compared to the 150mm x 100mm track they run on.

I do appreciate your concern though.