Needle thrust bearings

[cncuser1]

I am in the process of repairing a homemade knee milling machine.

It is about 80% smaller than a sieg X2. Made from castings and fairly tight and rigid.

I am in the process of trying to reduce backlach in the ballscrews ( 3/8 nominal dia with .125" lead).

my question is as follows:
The previuos installation uses two sets of clamped up bearings ( see attached picture) I am wondering if they are sufficient?(The radial play is held by a bronze bushing)

It seems to be rigid enough, but I wont know till it is all re-assembled.

Anyone hazzard an opinion?

[NC Cams]

You can determine the "sufficiency" of the bearing to absorb thrust by measuring its dimensions (or pulling the part number from it if one can be found) and checking the Torrington catalog for the static thrust capacity of the item once you identify it.

If you don't know the potential thrust that will be applied by your machine via the screw, your guess is as good as anyone's as to the ability of the bearing to absorb thrust.

[handlewanker]

Hi cncuser, if you're using a 3/8" leadscrew then you wont be transmitting any significent force to worry about. I'd be more concerned about the fact that you are using a bronze bush with two thrust washers to do the job of end location. A better solution would be two angular contact bearings in back to back configuration which will do the complete job and if sealed in a housing would take care of lube problems as well.

[cncuser1]

Wanker...
I didn't go into too much detail abouut the bearing arrangement but I have a 3/8" ( 10mm) thick piece of steel that is sandwiched between the two bearings. One bearing is on eitherside of the steel. They squeeze the piece of steel, by a 1/4-20 (6mm) nut. The bronze bushing is press-fiit into the steel. So the bronze acts to contain the radial loads only. I am not too worried about the bronze.

That axial loads( in both directions) are transmitted via the bearings to the steel. each of the bearings are sandwhiched by hardend washers.

[handlewanker]

'Allo cncuser, the bearing arrangement will work Ok as you've described it, personally I always avoid bronze bushing as it has problems that cannot be forgotten "in the heat of battle" so to say. They generally wear away unless the lube is kept up and then want pulling apart to start again. Sometimes though it is the easier way to go, especially on slow moving items like control rods and levers that only move a bit.
Where you wouldn't use them is if you had gears attached driving an adjacent gear, as any radial wear throws the gears out of mesh and gives backlash.
I take it that this is a ball leadscrew arrangement, direct driven by a stepping motor, so no radial forces are involved.

[Rich Carlstedt]

Needle Thrust bearings are good high load capacity bearings.
The work well with "precision ground washers" (only).
The problem they have is preload. Angular contact bearings sneak up to preload levels, while needle thrust are almost "all or nothing' becaus ehtey are totally in a single plane (at right angle to ball screw)

If you can present a 'square' loading nut to them, and maybe a precision method of adding load, they will work well.
Most designers shy away, because the length of the 'square nut' is far longer than the normal angle contact bearing requires, so space is an issue.
hope this helps.
Rich

[handlewanker]

Hi cnuser, I just came across this needlebearing/angularcontact combined bearing on the SKF bearing website. It is a regular needle bearing but also has a small angular contact bearing combined with it to cater for thrust. Main attributes are a very low profile. The bearing no. is SKF nkia 5908.
I think this will prove to be very cost effective, as well as the need for a low profile bearing package. It will require to be run on a hardened shaft, but where soft shafts are used then a hardened sleeve is used and the next size up is required. Hope this is of use to you.
Ian.