Tolerance for needle bearings?

I wonder whether the problem is the threaded rod rather than the rollers.

18mm diameter seems quite large and need quite a lot of torque to rotate it, I presume the diameter was chosen to avoid buckling.

I would have thought a 12mm diameter screw would be more than adequate if its is in tension. It could be driven from the bottom but the thrust load on the screw would have to be taken at the top (so the screwed rod was in tension) and have a decent thrust race.

Ian P

HI Gary, take a look at these, I bought a similar one for my Mother in law, but this looks like it does exactly what you need. I may be easier to adapt it with a base for ordinary floors.

Bath seat

Neil

It would be doubtful if the design with a single screwed rod could be made to work reliably, unless the load is spot on for balance the platform will cant over and jamb, you really need two screw jacks geared together, or a pair of winched, which ever they should be placed as far apart as you can get. You might get away with 4 skateboard wheels, bottom ones on the seat side of the pole, and the top on the outside. There is enough material on the wheel to fit larger bearings and still take the outer diametre down a bit.

A jack arrangement that  I helped design can be seen on the draw bar of this machine for feeding out hay, it will take a load of two or three tons at least. It consists of a 50 sq tube sliding over a 40 sq tube, there is a 24 mm nut welded in the 40 sq and a bit of 24 mm threaded rod down the middle. The jack is designed to be farmer operator proof(almost)

Ian S C

Edited By Ian S C on 03/08/2019 12:51:24

I originally used M12 Allthread, but it vibrated so much that I concluded it was too thin - hence M18.

I tried precisely that chairlift Neill, but apart from being rather unstable, it had two insurmountable problems:

1. In its collapsed position it was too high - the guy couldn't get on it.

2. It couldn't lift high enough.The seat of the wheelchair is 23" above floor level.

The part number of the bearings I've purchased is HK0910 (13x9x10).

I'm pretty much convinced that friction is the enemy here (comments are welcome). Would an Acme thread be any better than a standard metric? What size rollers would be more suitable, bearing in mind that bigger rollers increase the seat distance from the guide rails?

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What I had in mind, Gary, was something like hard skateboard wheels ... but I didn't want to suggest any specific item, size, or proportions.

For what it's worth: I built a device in about 1972 that used the geometry that I am suggesting, and it worked beautifully. [Note: mine was dead-weight counterbalanced, and lifted by hand; so I have contributed nothing to the discussion about threads]

MichaelG.

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https://www.warehouseskateboards.com/help/Skateboard-Wheels-Buying-Guide

Edited By Michael Gilligan on 03/08/2019 13:43:04

Ian, I'm not convinced that two screwed rods would make the seat any more stable than the pairs of rollers that surround the uprights, as shown in the picture. Am I wrong?

I've removed the parts that obscured the view.

For that application look at the track rollers I linked to in my earlier post

https://www.bearingboys.co.uk/SupportYoke-Track-Roller--1724-c

I also think you need two back braces either wire, rod or tube, one on each side to stop the frame twisting.
While I know Remap provide liability insurance, how do they deal with CE marking? As a powered lift for human this arguable comes under a whole raft of legislation. I'm not saying don't do it, just curious.

Robert G8RPI.

I don't think two screws are automatically better and you would have to use toothed belts (or encoders and feedback if you are of an electronic bent) to keep them syncronised.. One thing to consider is rollers or bearings of some sort on the outside faces of the frame. unless the weight is exactly centered the moving part will try to tilt sideways and as there are just flat plate it will lock up. The don't have to take the same load as the main rollers.

Robert G8RPI

Edited By Robert Atkinson 2 on 03/08/2019 15:36:23

Not sure if this will be of any help, but here goes anyway :

Some time ago ,I built a workshop hoist after the style of one made by Dave Fenner some years ago.

This design made use of U shape sections for the uprights, with the rollers within the frame and the table/seat overhanging to one side. This unit uses a wire rope winch to raise the table but there is no reason why it might be power driven. My hoist seems to work quite well although the rollers are just plain bar.

The main point is that the support sections do not flex in any direction even under 50kg loads.

I do not claim that this is my idea, but it might be worth looking at. I think there is a picture of it in my album

Michael makes an excellent point about skateboard rollers. Readily available, proper bearings, more than up to the load and all the pesky details sorted so they just work.

Size isn't an issue if you narrow the seat bit slightly so the back portion sits inside the frame. Plenty of room for wheels then.

Your drive system appears to show the motor at the bottom with the screw working in compression. Wrong way round as it will try to buckle giving a bending / whipping effect that will send nut friction through the roof. The screw must be hung from a bearing at the top so its in tension. Bearing needs to be fairly free and the nut in a pivot so it all takes up its natural position. Look at the old style car "plug in the sills" side-jack. Engineering standards are applied crudity really but they lift half a ton with no great trouble.

The original EasyRizer used a simple Allthread screw of similar dimensions to what you are using and was frequently demonstrated with a Honda Gold Wing perched on it running up and down under drive from a fairly ordinary, albeit good make, battery screwdriver. If yours has so much friction that it won't lift a person you have some serious fundamental design issues.

I'd scrap the "door frame" and go for a single column with two triangulated braces back to the rear foot extensions. Picture shows the guide roller end of a hydraulic lifter rated for something over half a ton on a platform similar to, but rather larger than a person seat. Perhaps approaching double the effective overhang.  Weight limits set by the hydraulic pressure and base details not column & guides.

Main tube is 2" by 3", axle pins are about 3/4" diameter on approximately 5 1/4" by 4" centre. Rollers are about 1 1/4" diameter with a 2" (ish) flange on one end. The other flange is free running. No significant side to side wobble when loaded although flanges just barely don't touch the main tube.  I imagine there is reason for one flange on the roller and one loose.  All steel. Made to rusty industrial "keeps on going" standards. The actual lift is between the lift guide tube and the platform. A top hung screw would work just fine. That one has a simple vertical hydraulic cylinder in a tube which connects the top of the hydraulic rod to the guides and platform. Tube also protects the rod when extended. For a screw version simple overlapping covers would seem more than adequate to keep things out of the works

Always remember being told many years ago when struggling to make one of my genius ( aka stoopid) ideas work "When your are in hole stop digging and copy something." Advice I was young and hung ho enough to ignore at the cost of a weeks worth of spare time to end up with less than stellar, albeit more or less functional, results.

Clive

Edited By Clive Foster on 03/08/2019 15:43:46

Edited By Clive Foster on 03/08/2019 15:45:23

I might be misreading the drawing, but there looks to be 4 rollers. I'd have expected there to be a roller on the seat side at the bottom, if not then the seat is skidding on the pin, you need rollers on the seat side at the bottom and the back at the top

You can get cam follower bearings which have a thick outer race for this type of application. In my previous life I used to design lifting equipment. As it's for REMAP, if you want any sums doing/checking give me a shout

.

I read it as eight rollers, Duncan [but perhaps I was wrong]

... Significantly, however, I think we reached the same conclusion as to what is needed.

MichaelG.

There are eight rollers - 4 on each side. I've made the seat semi-transparent to make them easier to see.

What are cam follower bearings and what sizes are available.

Incidentally, I intended to use two needle bearings to replace each of the four Delrin bearings that actually take the load - the Delrin ones are 20mm wide and the needle ones are 10mm.

Cam followers are similar to the track rollers I liked to previously but typically have a slightly crowned otter surface

**LINK**

Have tried applying load to the sides and front edge of the seat to see if you are getting some kind of locking up? A load directly on the cross member with the nut will tell if it's the screw that the issue.

Do you have a proper thrust bearing at the bottom of the threaded rod?

If you can put an ammeter in series with the motor that will give you a direct indication of the motor torque and you can see the effect of putting the same load in different places.

Robert G8RPI.

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Thanks for confirming that, Gary _______ As pictured there ^^^

... upper-left and lower-right are doing the work

... upper-right and lower-left are only acting as check-straps

It should be a simple matter to use appropriate items in each position.

MichaelG.

deleted

Edited By Sam Longley 1 on 05/08/2019 08:25:59