We recently installed two Becker vacuum pumps on a 20' table. They combine at the pumps to a singe 1 1/2" line that runs to the table and is about 75' long. We seem to be having some trouble holding material down to the table through the MDF. The meters on each pump are reading around 16 oz. of mercury which doesn't seem too low, but was wondering if anyone had any solutions to help with holddown or with getting more pressure from the vacuums. Is the airline sufficient at 1 1/2"?
From Contributor H:
It sounds like you have a vacuum leak somewhere. Are these pumps new? If they are used you may need new vanes installed. This can cause them to have a weak vacuum. They should be drawing about 29" of hg. If they are unable to draw this with the valves closed, then that will lead you to a cause from the valves to the pumps. Either the valves aren't closing fully (there is a leak going to the pumps) or the vanes are worn.
If the valves are closed and you are able to get that much vacuum then I would start looking from your valves forward. You can have a leak somewhere if the system or the board you are using is too porous. You can try painting the edge, or edge banding the board to seal up the edges. This would prevent any vacuum leaks through the side of the spoil board. It is also possible to draw vacuum through the spoil board and your sheet. You can test this by placing a piece of paper down on top of your sheet. If the paper sticks, you are drawing a lot of air through your spoil board and sheet. You will need to use a less porous spoil board on your table. This will restrict the vacuum and hold it to the sheet.