View Full Version : Drawbar - Belleville Stack selection

09-30-2017, 11:32 AM
Looking for some help/advice in selecting Belleville washers for a power drawbar. I'm looking to use tormach TTS tooling and trying to achieve over 2200lbs of force. Drawabr is R8 so the ID needs to suit a 7/16 UNF (approximately 11mm) drawbar and the OD of the spindle is 36mm.

I've been looking at the S4308 washers in Ondrives https://www.ondrives.com/data/pdf/bearings/spring-washers_shims/s.pdf

These are 2719N (611lbs) and have a loaded height of 1.75mm

From the diagram I take it the loaded height is how far I can compress the washer - i.e. 1.75mm (0.069") of travel.

If I stack them 4 high, then I get 10876N (2445lbs).

Are my assumptions correct? Stacking 4 washer together seems far to simple compared to other peoples setup!

09-30-2017, 11:32 PM
1. Figure out your desired clamping force. (>2200lbs)

2. Figure out your minimum travel to release the TTS. I figured on .125" when I built my PDB, but really you only need about .040" unless the TTS shank or collet are super-cruddy. Go with .050" or .075" to be safe.

3. Loaded (15%) height for the S4308 is 1.75mm. Spring is 1.5mm thick. Available deflection to flat is thus .25mm (.009"). Because you are stacking them like (((( and not ()(), then you'll get .009" of travel until flat for the whole stack. No dice on a 4x1 stack, but a 4x5 stack of these will get you 2445lbs and .045" of travel.

If you look at the Tormach PDB installation manual, they have (I think) 4 springs stacked ()(). Each spring has a much higher rate than the S4308 - they're around 3mm thick. I think Tormach preloads their TTS less than the 2200lbs you indicated - but like you (I suspect) I didn't want any pullout. Ever.

4. What you really need is the spring rate, because your preload may be good to go at 2400lbs, but if the spring force is too high for your drawbar actuator at the release distance (air cylinder, whatever) then you can't compress them enough to release the tool. Go to Key Bellevilles and sign up to download their spring calculator spreadsheet. It's awesome - lets you play with different stack heights, arrangements, spring rates & thicknesses and so forth.

5. How long do you want them to last? The shorter each spring has to travel to achieve release the longer it will last - in theory, of course. Think of .040" spread out over a (((( stack of four, versus (((())))(((()))) and each one moves less for the same travel.


10-06-2017, 04:20 PM
Sorry for the delay in replying, have been away with work. Thanks for taking the time to reply. Some great info and I'd forgotten to remove the disc thickness from the travel.
The challenge I've got is that I'm really limited on space - some pics here (http://www.cnczone.com/forums/benchtop-machines/344382-cnc.html#post2090296

There won't be enough space to put a collar around the bellevilles, so that limits the ID I can use as I will need the drawbar to be the guide to hold the washers in place.

10-06-2017, 05:23 PM
You've got room.

What I'd do is make a cup washer as the bottom bearing plate to sit on top of your spindle. Should have a lip on the bottom to keep it centered on your spindle, and be reasonably thick so it doesn't deform under the spring pressure. Bore should be larger than the top bearing plate spigot ( see below).

Next, add your stack of springs - shoot for a 4x3 or 5x3 stack like I mentioned. The top washer/bearing plate should have a long sleeve/spigot on it that fits around your drawbar and inside the spring stack for the guide. In fact, you could measure the uncompressed stack height and thread a piece of bar stock for the drawbar (draw-bolt?) then turn most of the length down as the guide spigot. This spigot also needs to pass through the bottom bearing plate, so a taper on the lower end of the spigot would be your friend here.

Now you've got a stack you can torque down and still have guided travel and enough throw to release the TTS shanks.

Consider using an air cylinder with a lever off-set from the spindle to press down on the stack. With enough leverage you can compress the stack while avoiding the necessity of running super-high air pressures or needing a huge 120mm air cylinder (or both).

Activation time will be longer than if it were direct acting (since the cylinder stroke will be much longer), but do you care about a 2 second release time vs. a .25 second on a hobby mill?


10-07-2017, 07:00 AM
I am actually looking at something different to air - I am going to try an electric actuator. I got a 6000N (1350lbs) electric actuator for less than the price of an air cylinder. The actuator is rated at 5mm/second so I hope it will make a happy medium.

Out of interest my other challenge is attaching something to the spindle to pull against so I don't put excessive load on the spindle bearings. I am thinking of drilling and tapping the top of the spindle with 4 x M5 or M6 bolts that go through a collar into the spindle. I see other like to use the thread that their bearing locknuts are fitted to, but I cannot get to mine.

10-07-2017, 11:59 AM
Normally I'd support your inclination to have something to pinch the spring stack, but the electric actuator is going to be really slow. I think your bearings are capable of withstanding significantly higher loads than the PDB can impose as long as you don't shock them. The slow actuator won't 'bang' the bearings and dent the races.

If you're really keen to do something, you could pull the top cover plate and open the spindle hole up to gain some radial clearance a threaded or clamped flange on the spindle top.

10-08-2017, 06:47 AM
Hmmmm - you've got me thinking now....

As it happens I am changing my spindle bearings. My mill is now 33 years old and the spindle started to feel a it notchy a few months back. I decided to change the bearings and also fix the oil leak it has always had on the auto-lube system. Of course that has evolved into a full rebuild, new bearings, new stepper motors, new VFD to replace the 30 yr old one etc.

Anyhow, back to the bearings, I got pair of Naachi 7208B-CDUP4 bearings to fit. Calculating axial load capability seems quite complicated, but a general figure seem to be 30-50% of the basic load rating. In the case of my bearings, basic load capacity for the pair is 59500N, which at 30% axial load equates to 17850N - so plenty more than my propose drawbar load.

At the end of the day my machine is only a hobby machine so it is not going to see a massive amount of use.

I've found a good supplier of bellevilles in the uk, not the cheapest, but they seem to have a good range - Belleville Washers Search Results (http://www.leespring.com/uk_browse_catalog.asp?springType=W) . Just waiting for their catalogue to arrive to see what combination of bellevilles might work.

Thanks for the ideas, great to throw some ideas around and see what comes out of it.

10-08-2017, 04:22 PM
If you haven't already purchased a VFD, you might consider switching over to an AC servo for the spindle motor. They're generally smaller than the equivalent HP 3 phase induction motor which may gain you some desperately needed real estate for a PDB assembly.

DMM Tech has a 5kRPM servo & drive combo that looks pretty nice. They also have a higher-HP model with a 3k max speed, but that can be solved with pulley ratios. Both drives take 220v single phase input. Looks like about $500-$600 American Bongo Bucks.

Obviously, this depends on your controller software and hardware - if it can't output step and direction for the spindle, then the servo will still work but you'll lose the ability to do spindle positioning. However, because the DMM Tech drive has encoder signal pass-through you can use that signal for rigid tapping (again, if your controller can do this).

10-10-2017, 05:13 PM
I've already got the VFD, it came for free from a factory clearout. You probably won't believe it, but the spindle motor is only 0.37Kw !

10-11-2017, 05:26 PM
So if you've got no money tied up in it, sell the VFD and motor and get something with a bit more grunt. Like a 0.5Kw servo.

10-16-2017, 03:57 PM
Would like to change the spindle eventually, but with everything new motors and motion controller I cannot really afford it at the moment. Think I've found a suitable Belleville - Lee Spring (http://www.leespring.com/uk_product_spec.asp?partnum=5001001063&springType=W&subType=)

Stacked 5 or 6 high they will perfect