Tolerance for needle bearings?

I have some small drawn cup needle roller bearings - 13mm OD and 9mm ID.

When turning the axles I decided to turn them initially to 9.05mm and then creep up to the final 9mm, but the bearing is a decidedly slack fit at 9.05mm. Is this as expected? Should I turn them to 9mm, or redo them to make a good fit?

Gary,

Is this fitting with the bearings unfitted to their final positions? They may need to be pressed into a sub-thirteen mm hole and slightly squeeze the ID whilst doing that?

Have you fitted the bearing in to its housing yet. If not the shaft, at the correct diameter, will be "slack".

Drawn cup bearings needed to be fitted in to a housing of precisely the correct size and shape, this will bring the bearing to the correct size and geometry.

John

Check on a needle bearing manufacturers sight. Make sure you know exactly the description of the bearing. Limits and fits vary slightly according to the type of bearing and the housing material. With thinner wall bearings the fit has greater influence on the fit.

The needle roller bearing outers will not shrink in every application, such as in plastic wheels. The good manufacturers such as INA certainly have recommended interference fits for each bearing based on the bearing number. This is because simply measuring the OD is not as accurate as with a normal ball race.

Gary,

You are turning the axles to diameter. I have to assume the shafts are soft. When you do your research on the bearing fit check o the hardness requirement for the shaft. You will find that the shafts must be hard for proper performance.

John

Only if you want to put a load on them commensurate with their capacity. There are lots of locos out there with 'soft' axles running in needle rollers

Thanks for the information guys. That all makes sense. The outer race will not be supported - it will just roll along a flat tube where it will have to withstand a radial thrust of about 100Kg. I considered case hardening the axles, but in light of Duncan's statement am having second thoughts.

If the housing is "soft", you should make the shaft a wee bit on the large size. The correct fitting is a light press fit in the housing as quoted above.

There was a long discussion in ME probably in the 1990s, and in the end someone(seemed to know what he was talking about) decided that on a model loco with unhardened shafts the bearings would outlast the loco, I think the material suggested was silver steel, but I think ground cold rolled steel was also mentioned as suitable.

You can get steel sleeves that fit on the shaft to act as the inner race.

Ian S C

If you are literally using the outer of a drawn cup needle roller bearing on a flat surface like a tyre you will have trouble. The walls are not designed to be run un-supported. Running them un-supported will either roll the outers oversize or cause them to fatigue and breakup depending on the material and hardness. Have you considered using bearings with machined rings or even better a track type bearing?

**LINK**

You might get away with it at light loads but 100kg is not light.

Robert G8RPI.

Impact load, not rolling load carrying, is the big issue with relatively soft shafts.

Its essentially line contact between roller and shaft so local forces can be very high indeed if things get clonked. No great issue when you have a shaft hardened to support many years of continuous use but a soft one, although well up to a couple of thousand or so hours of undemanding use, is vulnerable.

Hopefully not germane to this application but the needle roller bearings in the original BMW K-series "flying brick" motorcycle clutch lever had a different take on problems with using unhardened shafts in needle rollers. Sealed for life bearings in an alloy actuating lever at the back of the gearbox just in front of the rear wheel with little protection from mudguard and swinging arm. As usual the sealed for life bit didn't last well. Once the seals failed water got in and rusted the bearing cage. Expanding rust pushed the needles into the fairly soft shaft giving a set of splines so nice you'd have thought they were intended to be there. Bit like the groove in a used Velocette clutch thrust race.

Getting the now expanded shaft out without mulluering the alloy ears on the gearbox that were supposed to support it tended to be an adventure. Not helped by BMW's contender for the worlds most inaccessible (and functionally dubious) E clip competition. Back in the day I fitted grease nipples to the arms for a fair few folks. Normal practice seems to be to just wear out the gearbox ears once its all seized up. Made myself very popular during a visit to the BMW factory by rolling a pocket full of mullered shafts and bearings onto the table after a "BMW are Wonderful" presentation and asking if they had sorted that problem yet!

Clive

Sounds like a proper roller bearing, with a hardened outer race is what is needed.

Drawn cup needle rollers are intended to be supported by the housing. Assymmetrically loaded, and unsupported, the thin housing will distort and be liable to fail in fatigue.

Howard

Thinking the same, Gary W. (OP) do you have an ID/part number for the bearing you intend using?

John

I have a positive distrust of needle roller bearings since an experience with one back in the 60’s. I was the proud owner of a Triumph Herald Estate with only 28,000 miles on the clock, I backed it out of the garage one morning and it was ticking over nicely when there was a loud graunching sound and the engine stopped. On investigation it appeared that a gearbox fault was the problem so it was gearbox out and strip. The problem was that there was a needle roller bearing which was between the input and output gear-shafts, the end of one ran inside the other courtesy of a needle roller bearing. What had happened was the bearing had disintegrated and the rollers had exited from the bearing and fallen down between the gears rotating in the box, jamming it solid. It was a complete mess and a recon box was the only remedy, but there was a three week wait so I reassembled the box minus the bearing and the debris and managed to use the car for three weeks with no bearing between the shafts, just to get to work, I was not worried about doing any further damage as the box was effectively scrap anyway, surprisingly it ran ok although I didn’t push it hard. So I am very wary of the strength and desirability of needle roller bearings.

Dave W

Listen to what Robert Atkinson says about the danger if using the outer part of the bearing as a roller. The wall thickness is very thin and will fail very quickly. An outer sleeve of 3mm wall thickness would help for light use.

I replaced the pin bearings on a reloading press and they soon broke up. It seemed the links were of soft steel and distorted the hole with the pressure. Had to revert back to the pin again.

Here is the reason for the query. It's a Remap job to help a 75Kg paraplegic get himself up off the floor onto his wheelchair or bed etc. He has sufficient upper body strength to get himself onto something a couple of inches high, but no higher. The concept is a lift with a seat that is raised and lowered by means of a screwed rod that is rotated by a motor from an 18v cordless drill. The seat, which is only about 1" thick, is raised from the floor up to about 23".

The screwed rod is M18 Allthread, and the nut is brass. Although the motor should be able to lift the person up 23" in about 1 minute, it can't. We think there is too much friction. The original rollers were made of Delrin, but we think that they deform under load to create too much friction. We replaced them with brass, to no effect, so we thought to use needle roller bearings. Here are some pictures from the Fusion design.

.

A suggestion, if I may, Gary ... Re-engineer the roller assembly:

With the seat cantilevered, only the rearmost rollers at the top, and the forward rollers at the bottom actually do any work. ... That's fine [i.e. just as it should be], but you need to embrace the fact.

Replace those four rollers with nylon wheels, running on proper bearings, and the other four with simple rods [you can do this because they only act as 'check-straps' and don't actually carry load]

MichaelG.

As always with this sort of thing the devil is in the detail.

Delrin rollers should be OK. They work fine on the EazyRizer motorcycle lift which works on much the same principle albeit with single column upright and obviously stronger construction to cope with five times the weight. If I recall correctly from when I had one for about 3 weeks the rollers were rather larger than yours appear to be. If someone local has got an EasyRizer it might be worth looking at it for clues as to why yours doesn't work.

I recall coming across a reference somewhere to the relationship between roller size, loading and freeness of running which suggested that things go very nonlinear below about 1 1/2" diameter leading to much more friction than you'd expect if the design is little out.

I think the thing is twisting which will seriously increase the loads. Having hit that problem in a rather different area myself I think you need either 8 extra rollers to stabilise it in twist or to start over. That rectangular "doorframe" style upright is very flexible. A single, larger, column would be much better behaved.

I'd prefer a proper lead screw and nut to Allthread too.

Hafta say that the device reminds me of far too many episodes in the past where I "just built something simple to do an apparently simple job" rather than sitting down and designing it properly first. Result was always endless trouble getting it working properly. Easy to overlook simple but critical issues when your main attention is on the welding torch and chop saw! At 65 I'm finally getting a bit better but ....

Clive

Hi Michael, I'm not interested in being right, I just want to make a device that works, so I'm open to all suggestions .

In order to reduce the overhang as much as possible I designed the roller assemblies to be symmetrical; all 13mm basic diameters with cheeks or flanges as guides. The "jockey" rollers don't actually do anything except help guide the seat. The axles were originally 8mm diameter. By "proper" bearings I assume you mean ball bearings. I thought of that but was advised that ball races with an OD of 13mm wouldn't withstand the expected forces - that's why I thought of needle rollers.

Unfortunately, increasing the diameter of the rollers increases the overhang of the seat relative to the upright guidance rails.