Thread: Radial and Thrust Bearing Question

**lowslo** · 11-25-2009, 06:26 PM

So I'm building my second router and I'm interested in opinions about the use of radial and thrust bearings on the lead screws. My first machine used both types of bearings on both ends of each screw. My new design....well thats wherer I would like some opinions.

I'm strongly considering using two thrust bearings and a radial bearing on one end of the screws with the driven end being held only by the stepper.

From what I can tell this is common practice. My concern is that it may induce premature wear on the stepper shaft and or bushings/bearings (Nema 23).

I have Dumpster CNC couplers and it would make the design simpler if I chose to go this route.

Cutting area is 30" X 40"
10 tpi 5 start screws (X&Y)
Dumpster CNC Derlin nuts
400ish oz. steppers
I will be cutting foam, balsa, & light ply.

My first machine is about a year old and has a little over 300 hours on it and is surprisingly still pretty tight. I have a paying job that keeps it busy most of the time cutting foam parts and I need more SPEED!!! Time is money!

I don't want to wear out steppers just to try and save a few bucks, but if they are sturdy enough to do what I'm contemplating then why not make the design simpler?

Will the second machine still be capable of cutting heavier ply, MDF, etc with the more agressive screws?

Thanks for the advice and opinions!

**FandZ** · 11-27-2009, 05:01 AM

Originally Posted by lowslo

I'm strongly considering using two thrust bearings and a radial bearing on one end of the screws with the driven end being held only by the stepper.

From what I can tell this is common practice. My concern is that it may induce premature wear on the stepper shaft and or bushings/bearings (Nema 23).

From what I've read, it's a bad idea. Now I have seen builds where you have the stepper, coupler, bearing block of your choice and the other end is hanging free. Just recently I saw a heck of a solid mill type cnc built that way. I'd avoid what you are suggesting though, especially since you have paying jobs to take care of. You don't want any down time.

**Turbo1zz** · 11-27-2009, 05:35 AM

You want to support the lead screw as much so that you are not putting lateral pressure on the motor shaft as it will cause the motor to wear out prematurely. Support the lead screws with bearing at both ends and use some sort of collar or flange to keep the lead screw from putting any kind of force on the motor. I have mine setup with the motor attached using a lovejoy connector then a shaft collar against a bearing then to a bearing on the other end and another shaft collar to hold the screw in place. all you want the motor doing is turning the screw and nothing more.

**lowslo** · 11-27-2009, 02:36 PM

Thanks for the advice.

I'll order some LoveJoy connectors and use radial bearings at both ends of the screws. I guess it would be acceptable to put the thrust bearings on just one end of the screws?

**jsheerin** · 11-30-2009, 05:39 AM

Put the thrust bearings on the end of the screw near the stepper motor along with a radial bearing. The other end will have the radial bearing for support but will be free to 'float' along its length. When temperatures change, this will let it expand and contract without binding things up.

**tobyaxis** · 11-30-2009, 07:37 AM

For my VMC design these bearings from NADELLA are going to be used.

I was going to use the Servo (your using a Stepper) Shaft but decided to Add more Rigidity.

http://www.tracepartsonline.net/%28S...2012001-127505

Also being used will be the Wide Low Profile Linear Rails and Guides.

They can all be found in the Trace Parts Library.

**lowslo** · 11-30-2009, 05:33 PM

Thanks guys for your input!

Neat bearings tobyaxis!

My plan now is to machine the bearing blocks to accept two stacked radial bearings on the driven end of the screws with a thrust bearing on each side and use a radial bearing on the free end of the screws. The bearing blocks will be an integral part of the machine end plates and not a seperate part.

Already have them drawn up and have a few parts cut!

Thanks again!

**FandZ** · 11-30-2009, 09:05 PM

Originally Posted by lowslo

Thanks guys for your input!

Neat bearings tobyaxis!

My plan now is to machine the bearing blocks to accept two stacked radial bearings on the driven end of the screws with a thrust bearing on each side and use a radial bearing on the free end of the screws. The bearing blocks will be an integral part of the machine end plates and not a seperate part.

Already have them drawn up and have a few parts cut!

Thanks again!

Be sure to post some pictures once you get your bearings and such settled in.

**ger21** · 12-01-2009, 01:40 AM

Probably too late, but you can use two angular contact bearings on one end and a radial bearing on the other end. The angular contact's handle both thrust and radial loads.

**tobyaxis** · 12-01-2009, 06:58 AM

Originally Posted by ger21

Probably too late, but you can use two angular contact bearings on one end and a radial bearing on the other end. The angular contact's handle both thrust and radial loads.

SKF makes a wide variety of Angular Contact Bearings that handle exceptional loads. LOL, I just like to over engineer everything.

http://www.linearmotion.skf.com/en/0..._bearings.html

**lowslo** · 12-02-2009, 03:34 PM

Your right it's too late! I did look at angular contact bearings and thought about using them but I already had a few regular thrust bearings and decided to go that route. I think on my next build I will use them though as they would simplify the design quite a bit.

I'll post up some pics of the parts I have now. I've got the Gantry sides and Z axis end plates cut out and hope to have the X end plates done tonight. Once I get the parts for the table done I can start a mock up.

**wildwestpat** · 12-02-2009, 05:05 PM

Hi Lowslow

You can simplify the support bearing even further by using one of the double row angular support bearings from SKF amongst others. These consist of two AC bearings in a single housing with an internal pre load. These only require a simple threaded retainer for the screw and a housing that retains the outer of the bearing properly. The cost is also very reaqsonable as they appear to be on the prefered bearing list for several manufacturers. Example 3201 2 RS is a 12x32x15.9 mm. I have been using some unmarked ones for some time and there is absolutely no detectable end float in the screws which is good for bearings costing less than $10.

Also you may care to note that NSK specify normal radial deep grove bearings for their ballscrews. For the very small screws they specify flanged radial ball bearings (see the data sheet for the RMA 0801 5C7S screws. Two flanged bearings being trapped by the securing nut on the lead screw which clamps the bearings into a simple bore. A very neat solution and one I ran into when searching for a ballscrew for the compound slide on a 7x12 lathe where there is little room if the screw is to be in the same possition as the manual screw.

Hope this helps some time!

Regards

Pat

**lowslo** · 12-04-2009, 03:24 AM

Wildwestpat, Thanks for the info. I'll definately keep that in mind. Somehow I don't think this will be my last build!

Pics as promised.

Closeup of bearing bore in X axis end plate

X Axis end plate

Ganrty side with bearing bore and Y axis rail mounting holes

Delta box screwed and glued together

Bearing adjustment detail

End plate moch up

The bearing assembly will go like so:

Coupler - thrust bearing - end plate - radial bearing(s) - fender washer - thrust bearing - lead screw coupler - lead screw.

I'll post a pic of the actual assembly when I get to it.

**lowslo** · 12-05-2009, 01:53 AM

I got my Lovejoy couplers today, and I will have to say I was a bit dissapointed to find that they are not real tight. I didn't get the Buna rubber but the next hardest spiders as that was all that was available for the 50 size couplers. You can feel a slight amount of rotational play in them. Its not much and it might not even show up in the cuts. I guess I'll use them and see what happens.

**wildwestpat** · 12-05-2009, 11:08 PM

Hi Lowslo

I have had this problem with the Oldham / Lovejoy couplings in the past. Also with the grove being machined too wide and one one batch off centre as well! In every case I have sought and got repalcements. The coupling cross element should be a tight fit in the slots in the metal parts.

Suggest you ask for replacements as it is the expensive metal bits that are going to be up the creek.

Regards

pat

**lowslo** · 12-06-2009, 07:57 PM

A few more pics including the bearing assembly and arrangement. I'll have to say it sure is nice to design and draw up something a tad complex like this and cut out the parts and have them fit PERFECTLY! It is satisfying. I only had one slight problem. The X axis nut mounting flange was a bit too tall. I need to take it to work and mill off about .060" to put it where it should be.

As this machine is being built to predominately cut foam I made it as light as possible and tried to ad strength where I thought I would need to. I want to keep the mass of the moving parts down to a minimum so I can use a moderate amount of acceleration.

**lowslo** · 12-06-2009, 08:00 PM

I did find that the Y axis rails were a bit wobbly so I added this box structure to the rear which stiffened it up dramatically. It will also make a great place to lay my cable carrier!