TORMACH POWER DRAWBAR

[BIGBLOCK1965]

There is a thread on another machine group about the Tormach PDB. The poster is offering a kit to fit the PDB to another machine. The design looks very simple, with an air cylinder mounted over the end of the drawbar which presses down on a stack of belleville washers which hold tension as well as releasing when compressed. However, it seems to me that this would put a lot of pressure on the top bearing of the spindle each time it was engaged. The poster claims that their PCNC 1100 hasn't had any bearing problems due to this. Just wondering if anyone has comments before I commit to one. BTW- my machine uses double row angular contact bearings top and bottom on the spindle. Here's a picture of their machine setup.



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[SCzEngrgGroup]

It would appear the guy who made that kit did not take the time to understand how the PDB is supposed to work. The PDB is supposed to be allowed to move vertically, so the air cylinder squeezes the springs WITHOUT putting any load on the spindle. He really should look at how it works on the Tormach, and "fix" his kit.

That said, I very much doubt the 2500# static load the PDB generates is enough to do any harm at all the the bearings, unless they are already on their last legs.

Regards,
Ray L.

[BIGBLOCK1965]

It would appear the guy who made that kit did not take the time to understand how the PDB is supposed to work. The PDB is supposed to be allowed to move vertically, so the air cylinder squeezes the springs WITHOUT putting any load on the spindle. He really should look at how it works on the Tormach, and "fix" his kit.

That said, I very much doubt the 2500# static load the PDB generates is enough to do any harm at all the the bearings, unless they are already on their last legs.

Regards,
Ray L.

Thanks,
He says the picture is of the standard Tormach PDB on the 1100 machine- Are you saying that his picture is not a Tormach PDB? It struck me odd as well, because the Tormach unit sells for 1200.00, and this appears to be nothing more than an air cylinder mounted over the drawbar- hardly worth that kind of money.

[SCzEngrgGroup]

As I said, the Tormach PDB "floats". When the air cylinder actuates, the piston moves down until it contacts the top of the drawbar head. Once that happens, the whole PDB assembly rises up, until the plate at the bottom contacts the underside of the large black "washer" under the springs. Motion beyond this point compresses the springs between that washer and the piston, putting NO load whatsoever on the bearings at all The photo does not show enough to be able to tell whether this mechanism is in place or not.

Regards,
Ray L.

[BIGBLOCK1965]

As I said, the Tormach PDB "floats". When the air cylinder actuates, the piston moves down until it contacts the top of the drawbar head. Once that happens, the whole PDB assembly rises up, until the plate at the bottom contacts the underside of the large black "washer" under the springs. Motion beyond this point compresses the springs between that washer and the piston, putting NO load whatsoever on the bearings at all The photo does not show enough to be able to tell whether this mechanism is in place or not.

Regards,
Ray L.

Ray,
Thanks, after reviewing some other photos of the Tormach PDB, I can see the disc in the picture that the base rises up against, so I think this is the Tormach unit with the " no bearing pressure" design. If Tormach doesn't want direct pressure on the bearings, then I would not argue with their engineers and will go with that style myself.