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| Thread ID: 103343 | 2009-09-19 06:59:00 | Vacuum question | prefect (6291) | PC World Chat |
| Post ID | Timestamp | Content | User | ||
| 811874 | 2009-09-19 06:59:00 | Mate has old bus which has vacuum operated cylinders which open and close the doors. He has put another engine in with an air compressor and hooked it up with an air circuit for his exhaust brake. Can he use compressed air on the door ie work back to front on the vacuum cylinders. Reason is he loses a lot of vacuum from the leaky cylinders and its vacuum not available to the brake booster. |
prefect (6291) | ||
| 811875 | 2009-09-19 07:09:00 | Compressed air is a lot more - er, how to say this, volatile for that sort of application . Vacuum is a much more gentle force to use as it doesn't really depend on total system integrity like compressed air does . I suppose you could reverse the lines and MAYBE the actuator will work, but not for long as it's not designed for pressure, but vacuum . It's prolly do-able with a lot of frustration and re-engineering . But remember that the seals are totally different for vacuum and so are the control valve(s) . Personally, I'd try to reseal the cylinders for vacuum and stick with the original design . |
SurferJoe46 (51) | ||
| 811876 | 2009-09-19 07:14:00 | The seal on cylinders is only a rubber diaphragm, the control valve is just a 3 way tap ie vacuum in valve and port a and port b. 2 cylinders pull the door each way. Its just metal to metal insides. Could he use an air regulator to cut the air pressure from 120 psi down to some pressure that wont crush a person into the shape of a hacksaw blade if they are caught in the door. |
prefect (6291) | ||
| 811877 | 2009-09-19 07:15:00 | dunno off hand. depends on how the system operates. ie uses vac to open as well as close. if so switch the hoses around and feed it compressed air. i would say it a case of try it and see. i would think he would need a regulator to drop the pressure, system compression will be to high. hopefully he fitted the compressed air tanks as well. | tweak'e (69) | ||
| 811878 | 2009-09-19 07:24:00 | The bi fold door has an arm on it one cylinder pushes it closed other pulls on arm and opens it. Control valve just ports vacuum to either cylinder their is no neutral possy. Not sure if has air reservoir but I guess he would to get stable air for the exhaust brake. |
prefect (6291) | ||
| 811879 | 2009-09-19 07:34:00 | The seal on cylinders is only a rubber diaphragm, the control valve is just a 3 way tap ie vacuum in valve and port a and port b . 2 cylinders pull the door each way . Its just metal to metal insides . Could he use an air regulator to cut the air pressure from 120 psi down to some pressure that wont crush a person into the shape of a hacksaw blade if they are caught in the door . Workin' hard to get a mental image of a person in the shape of a hacksaw blade . Hruumph! PERHAPS if the pressure were down to . 2ATI or so, then maybe - but things vacuum are not happy being forcibly evolved to things pressure . The forces are obtuse and can upset the designs pretty badly . Try it and I'll watch the morning sky for the first two Kiwis in orbit . You and he should go over here on a suborbital pass once in a while I bet! Beep loudly for recognition . |
SurferJoe46 (51) | ||
| 811880 | 2009-09-19 08:10:00 | Maybe to upset the seers, you could strike a happy medium and set the regulator to 12 or 15 psi to simulate about the vacuum of an idling bus motor. I presume there is already a flow reducer to prevent door closing also shutting down the motor. If he swaps the hoses between cylinders, it might go adequately, with the lever working the same way it did in the good old days. | R2x1 (4628) | ||
| 811881 | 2009-09-19 10:27:00 | I was only an electrician but this is my thinking. I believe that the maximum vacuum is minus 30 lbs per sq inch. Is that right? So if the air pressure is no more than that, there should be no damage. However... the seals in the cylinder will be the wrong way round, won't they? The air will blow past the seal. The seal is designed for suction pressure not air pressure. PJ |
Poppa John (284) | ||
| 811882 | 2009-09-19 10:35:00 | I think 30 inches of mercury not negative psi. Maybe there was a flow reducer when it had its original petrol engine now it gets its vacuum from pump on alternator. I will ask Rex from Roving Repairs in Swanson he knows heaps about old stuff. |
prefect (6291) | ||
| 811883 | 2009-09-19 10:41:00 | I think 30 inches of mercury not negative psi . Maybe there was a flow reducer when it had its original petrol engine now it gets its vacuum from pump on alternator . I will ask Rex from Roving Repairs in Swanson he knows heaps about old stuff . Could be so . I am thinking that the amount of "Suction" (Vacuum) possible is much less than Compressed Air . Using Minus pressure for Vacuum & Positive pressure for C Air, suggests to me that the cylinder seal(s) are going to be the problem here . PJ |
Poppa John (284) | ||
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