Jerrys' belt drive gearboxs, (working)

[fozzyber]

Well I broke down and started making my own gear box belt drive system, for my old PRT.
xl belts, and bearing pivots...
It sounds like a different machine now, I can't jog over 7-8ips with out stalling I guess b/c of the pk-299 motors loose all of their torque at speed.
but at the lower end of things OMG!!! 2816 oz-in ( 176 in-lbs) !!!!!
Well pics can say more then I can.

[Brady Watson]

It really does make a world of difference. Good job.

Now that you've got way more torque than you'll ever use, you should upgrade a few things before you twist them into a mess or break them.

Get rid of gold strut on X car. Replace with box steel & weld into 3X3's on each end. Weld rails where they meet 3X3 on each end. This will keep the gantry from racking if you get a stall on one side or a motor driver fails - or you hit something.

Replace the 3/8" rack on the entire machine with 1/2" and 1/2" pinions. You're pushing what that 3/8" rack is capable of handling. It makes a tremendous difference in smoothness of movement and cutting over the 3/8".

-B

[donclifton]

What is the gear ratio, looks like a nema 23 motor. I think you math might be a little off.
Don

[fozzyber]

Motors are pk-299, ratio 3.2:1
880x3.2=2816

[donclifton]

Your right, those are some big motors. I have 500oz hold motors on mine with 5:1 gearing and it's way over kill. You rack and pinion takes at least 2/3 of that gearing away.
Thanks
Don

[fozzyber]

I can see that but this was done mostly for resolution than tourque. My cuts look much better now. Next project, is a box for the z,
I don't understand why the z does not have a pivot, I am thinking of designing a z with a pivot. Any thing to watch out for?

[donclifton]

You could always make a Z with ball screws.
Thanks
Don
4axiscnc.com

[Brady Watson]

The pivot is not necessary - in fact is undesirable, as indicated by the PRS way of motor mounting. The pivot allows the mount to introduce a lot of pitch & yaw into the mount and makes pinions wear out faster. Direct mount would be best.

Please be careful that you don't get your fingers or arm in a spot where it shouldn't be. The motor combo you have does not have safety provisions built into it & this can be dangerous. You literally have bone-crushing force at your fingertips...

-B

[fozzyber]

Brady do you have a pic of a PRS z motor mounting?
And yes I will be carefull with "the power'

[Brady Watson]

It's just a flat 1/4" plate that the motor mounts to, with slotted holes to slide the motor in & out of the rack. If you goto the shopbot main site, you can see how they are mounted on the tool. All axes are the same.

-B

[danhamm]

Nice job Jerry, with the bearings response to movement it should provide a good removal of gear vibration in your cuttings..

[br928]

Its seems to me that the spring and pivot design would insure that the pinion is always fully engaged into the rack. This is the first time I have heard that this design causes the pinions to wear out faster than direct mounting.

As pinions wear the spring design keeps the pinions fully engaged where with the fixed design the wear seems like it would causes some backlash and have to be adjusted.

With the fixed design I never understood how to consistently engage the rack and tighten the motors. I guess I questioned why the design was changed with the newer ShopBots. Are there other advantages with direct mounting I am missing here?

[Gary Campbell]

Stan...
I am not overly familiar with the spring & pivot mount, but have seen a good number of that type mounting, along with practical experience with the solid mounts on my 2 PRSa's. My thoughts on the comparison would be:

The single pivot and plate of the earlier design would allow flex under torque. The more torque, the more flex. As Brady says, flex results in misalignment and wear. The advantage of that system is that it compensated for wear or inconsistencies in the V rail. The newer style block mounted hardened rail and rack add the consistence needed to allow the solid mount motors. Solid mounting ensures proper pinion to rack alignment. The newer style requires a substantially higher precision in its parts.

The extra weight and higher center of gravity of the PRS gantry could not be held properly with the old system during high torque starts and stops.

I use the new system, and it works well. I check the adjustment once a month, maybe more if doing a lot of cutting. The only down side is that the pinion actually holds the V rollers into the rail. As cutting force is increased, so is force against the pinion and shaft. I havent heard of this being problematic, so it could be only my perception.

[fozzyber]

I can see and agree with the pivot causing mis-alignment. hence one of the reasons I beefed up my pivots with bearings and shoulder bolts. But with a fixed mount system what happens if you cut more in one area of the rack than anouther would you not Get binding in one area and slop in others with a fixed mount?

my current plans are to build a spring loaded gear box for my z. We shall see....

[Gary Campbell]

Jerry...
Its possible, but the rack is much harder than the pinion, so much less likely to wear. The rails and rack on the PRS are very parallel and in over 4 years of hard cutting, I havent noticed what you describe.

Also, if people were wearing racks unevenly, we would hear about it.

[Brady Watson]

My PRT has cut well over 750,000 parts over the years. It originally had 3/8" rack on it. I switched it out for 1/2" rack (36' of it...) when I upgraded to Alpha with 7.2:1 motors. I use the later PRT Alpha spec pivot motor mounts which are much stiffer than the original angle iron motor mounts it came with. Going forward, I am going to swap these out for solid mounts and ditch the spring. ANY flex is bad & every little spot you can tighten the machine up adds up.

-B

[bleeth]

The mild steel rails on the PRT are much softer than the hardened rails on the PRS series machines. Wearing of the rails from too much tension and/or overusing one area of the table is a reality. The force that is needed to "hard mount" the motors correctly on the PRS is greater then the force exerted by the springs on the PRT. Therefore it is conceivable that this increased force would have the affect of wearing the rails quicker.
Jerry-you have started down a road that I have been traveling for years since I bought my PRT Standard a few months before the first Alpha was introduced. First came Gecko drivers, then hardened rails, then the entire x-rail and gantry rebuild. As you know the next step was slated to be gearboxes and PK299-F45's and I have all the parts in hand. Then I picked up Ken's Alpha drivers so have set the 299/gear parts aside in favor of just going Alpha. The thing is that Brady's comments regarding beefing and welding the gantry are completely on target. The original PRT design just wasn't sufficient to really withstand a lot of torque. Hence even with the original standard motors and gecko box I could jog at 10ips plus but cutting was well under 2 to maintain a smooth edge.

[Brady Watson]

When I bought my machine, I was fortunate enough to have real angle iron rails - not the soft CNC bent rails that most 1999 or later PRTs had. They are much harder and stiffer than the bent ones. They have a squared 90 degree outside edge, not a rounded one like the bent ones. Since my rails are 18' long, it was too big to get into the bender...so I got angle iron all the way around. It wears so much better than the soft bent rails.

I don't think that eliminating the spring & rigid mounting the motors will cause any more wear to the rails than with the spring. Reason being - you are not cranking the pinion into the rack gear with a screw - you just lift the motor until is is engaged - then tighten the mounting bolts. There is no more pinion to rack pressure than you can exert with your hand and that the gear mesh will let you go.

-B

[gerryv]

18 f-e-e-t :eek: Now that you've moved to the farm will you use a plow to level that table :D