|
There are easy ways to do it, and there are hard ways to do it. 1. Hard. Use a lot of cheap components and worn out motors and swear a lot when it never starts. 2. Easy Buy one that is already built for far cheaper than you can make it. But, here I go anyway. First, starting. You can go with a centrifugal or electric clutch. Centrifugal is cheaper and easier and negates the need for a battery. Makes it a lot easier to pull start in the cold. Electrical is easy to disengege in an emergency, or for any reason you want to have the motor running without the pump turning. Pipe in a tee to the discharge line to the tank and put a valve with a small sintered metal muffler in it so you can blow the air to atmosphere while the motor warms up. Yes, you will still need a check valve. Ok, so you have the motor running and the pump is idle due to the clutch. Now you rev up the motor and the centrifugal clutch engages, or, you throw the little switch on the side of the pressure switch that the electric clutch is wired through. The whole lash up begins to pump. You of course are using synthetic oil in the compressor since they will experience a lot of wear running in the cold with thick crude oil based lubricants. So, you are coming up to pressure and now you want the pump to stop pumping. Ok, there are several things to consider: How do you do this? (duh)
How do I bleed air from the pump so it can easily re-start when I need air again? (unloading) The cheap and easy way to do it, and the only way to do it with a pump that does not have a centrifugal unloader or head unloaders, it to use a special bleed valve that simply dumps air through a muffler once pressure is reached. These can be fairly expensive, but are easy to plumb. There is a second port on the better ones where you can take air off to throttle down the motor (more on that later). Cheaper units simply let the engine rev up till its governor takes over. It works, but is not very elegant. The better solution is to use a pump with some kind of unloading system. A centrifugal clutch on the motor, with a centrifugal unloader on the pump, with a control valve to throttle down the motor is the neatest way I have seen gas units unload. First, the pump comes up to pressure and the pilot valve lifts off its seat and sends an air signal to the control valve on the motor to throttle down. I have seen a nice stainless steel Bimba air cylinder mounted on a piece of angle iron bolted to the motor controlling an 11 hp Honda, it works well and is very sturdy. The motor slows down to idle. The centrifugal clutch slows enough to disengage. The pump stops, just like in an electric application. The centrifugal unloader blows down the pump. You now have a tankful of air, and idling motor, and a completely still pump. You can stand next to it and talk normally. When the tire truck guys saw that, their jaws dropped. When the pressure drops, the air signal to the motor disappears, the motor revs, the centrifugal clutch re-engages and you are back in business. The most common way, and the best way if air demand is heavy, is to use a pump with head unloaders. It works basically the same way as the previous unit, but instead of stopping, the pump continues to turn without pumping air, as the intake valves are held open by the unloader claws. Usually, an air signal will slow the motor somewhat, as well as unload the pump, but not as much as it does with the centrifugally unloaded pump, since it still has to turn the pump over. With an electric clutch, simply wire the clutch through a pressure switch, so that when you reach tank pressure, it trips, breaks the circuit to the clutch, and the pump stops pumping. You still have to unload it however, and slow down the motor. Or, you could let the motor wail and use its momentum to get the pump spinning again despite having pressure in the cylinders, but this is really the neanderthal way to do it and very hard on all the components, especially the compressor crank. If you have a centrifugal unloader on the pump, it can blow down the pump, or you can bleed air off from the check valve back, if the pressure switch has a needle type unloader valve that you can tube into the check valve. You can have a 12V normally open solenoid air valve plumbed into the tank wired into the pressure switch as well, so that when the circuit breaks, it opens and sends an air signal to a throttle control cylinder on the motor to slow it down. All things considered, the centrifugal clutch/unloader method is by far the simpler way to go. Always have a safety valve in the tank. Always inspect the tank for cracks, since a gas motor will send a lot of vibration through the tank, much more than an electric drive unit will. There are two ways I have seen for slowing the motor. The most common way is to have a small air cylinder mounted on the motor that pushes the throttle shut once it gets the air signal from the pilot valve. The second is a throttle cable that has an air cylinder at the point where it mounts to the pilot valve. It pushes the cable in the sheath which is pinned to the throttle at the motor. This is a little easier to hook up, but you have to be careful not to kink the cable. Trimming it to fit can be a bit of a pain too. Thats all I can remember right now. I am going out for a coffee. I hate winter.
BTW, if you blow yourself up, I am in Canada and cannot be sued. Unless, of course, you are also in Canada, in which case, I am in America. Or not..
Seriously, if you do not know EXACTLY what you are doing, leave it alone. I am building Fisher pneumatic control valve actuators right now. I snapped a 1" steel shaft off the other day with only 35 psi of air pressure.
| 
