Hi,
vane vacuum pumps are good for low and even very low ultimate vacuums, but really thats not what you want for a vacuum hold down.
Lets say a vane vacuum pump can get down to 0.1 psi from atmospheric of 14.7 psi. If you had a 100 square inch workpiece te hold down force would
be (14.7-0.1)*100 =146 lbf. Note that 0.1 psi is about right for a single stage vane pump.
Lets say a liquid ring pump can get down to 1psi from atmospheric, its hold down is (14.7-1)*100=137lbf.
Really the difference of the hold own force between the two pumps is negligible. The real test comes when there is considerably vacuum leakage. The ultimate vacuum of the vane pump
will suffer badly. Liquid ring pumps however tend to have MUCH higher swept volume and the degradation in vacuum and therefore degradation of the hold down is less.
If you bought a HUGE vane pump the leaky vacuum will matter less, but otherwise a liquid ring pump offers superior swept volume and for less money.
Vane pumps are great when you need a really good ultimate vacuum, say degassing epoxy, or vacuum desiccation. For vacuum hold down with leaky vacuums
you need the highest possible swept volume.
Craig