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US Air Compressor MK Series

TxRzrBk

Plastic
Joined
Oct 16, 2014
I picked up an old US Air Compressor dual stage MK series pump with a 2 HP GE repulsion induction motor. I just finished mounting it all up to a SAFE newer certified 200 psi vertical tank. The compressor runs beautifully and sounds very smooth and quiet until the tank gets beyond 60-80 PSI at which point the motor starts making noise which looks to be the entire shaft and pulley "walking" back and fourth. I originally thought it might be a symptom that started once things were hot but I have aired it up and let it cool off and it does the same thing, I also purged the tank while it is hot and everything returns to smooth and quiet operation.

Not sure if it is related but I know this compressor originally had a centrifugal unloader but that appears to have been disabled, the small line connecting from the output line to the front of the compressor has been removed and it looks like there is a simple inline check valve presumably to account for the disabled unloader?

The flywheel seems to have a little wobble to it beyond 60 psi. I am guessing the pump itself could be the culprit with either worn bearings or something related to the unloader causing the wobble. Any help would be greatly appreciated.
 
Different, here is a picture

I have one in that shape. It is with a 5 hp motor. The unloader is mechanical and behind the four screw plate. Mine shucked the flyweights over a period of time. I attempted to make new ones after finding out part are not available. Finally gave up and installed an electrical solenoid in place of the mechanical device. Pump was so noisy I just knew it was trash. Turned out is was the broken flyweights making all the noise. I cleaned out the insides and looked at the cylinder walls and with the solenoid the pump is back in service and as quiet as most compressors. Mine is a model from the '40's or '50's, I think.
I have a blow-up if you need to see what the guts look like. PM me if you wish a copy.
 
How would I go about checking and balancing the flywheel?

I suspect the mounts to the top of the compressor might have a role in the wobbling also, it was mounted on some c channel steel which isn't as rigid as I'd like it to be, I might end up shoring it up with some angle iron.

As for the unloader, could someone give me a quick overview of how it is supposed to work and its purpose?
 
I have not balanced a flywheel but would do so by the static method on a shaft. As for the unloader, it is basically like a governor on a steam engine. Flyweights on a carrier which push a spring-loaded plunger in during pumping and allow the plunger to open the unloader when the pump shuts off. The pressure is bled off from the pump head to ease start up in the next cycle.
The parts blow-up I have does not show the flyweight assembly very well. None of those parts are available far as I know from my searching. Is why I attempted to make some coping the broken pieces. When about it the wrong way and when the pump started the "new" weight flung off with in 5 rpms I'd guess.
The electric solenoid system has been working just fine for a year or two now. The pump itself is in great condition considering it's age. It came out of a gas station built around the late 40's or early 50's and powered a car lift and a two cylinder truck lift plus the rest of that stations' air requirements. It sat out in the weather behind my dads' barn for several years before he & I walked it into the basement. I powered it up in '95 or so and it was a bit noisy then. I should have investigated at that time to find a flyweight laying in the housing causing the others to tap it and create the noise. Maybe two years ago it finally quit closing the bleed off during pumping and I opened the cover to find all the flyweights in the bottom of their housing. No real damage to the housing or pump.
Tough compressors in my opinion. Fairly simple inside, too. Big ball bearing on both ends of the crank. Rods are "end cap" style and could be rebushed if need be.
Only thing I'd change is to a larger capacity cfm for my needs.
 
I'd think the flywheel was balanced during the manufacturing process. What would have occurred to un-balance it? Have you removed the flywheel and put an indicator on the shaft to see if that's the problem? Is the flywheel a good fit on the bore? Any signs that the compressor fell over on its side at some time, possibly putting a kink in the flywheel?
 
The unloader seems unnecessarily complicated, would a check valve not achieve the same result?

I have not removed or messed with the flywheel at all. As mentioned everything seems smooth until the pressure is above 60. I am thinking the extra force required to spin the pump at a higher psi is exposing a weakness in the pump base mounts.
 
The unloader seems unnecessarily complicated, would a check valve not achieve the same result?

I have not removed or messed with the flywheel at all. As mentioned everything seems smooth until the pressure is above 60. I am thinking the extra force required to spin the pump at a higher psi is exposing a weakness in the pump base mounts.

Not the same, see: Unloader Valve
 
Thanks, I understand what the unloader does but still think the centrifigual unloader is over engineered. I believe an automatic tank drain connected between the check valve and the head would achieve the same result, if not a simple valve on the pressure switch like most modern compressors have would be easier than attempting to make the existing setup function. the unloader is clearly not related to my primary issue which is the compressor wobbling above 60 psi.
 
Sorry I didn't answer your query. It could be the bearings on either end of the crank, bent or loose flywheel, or maybe the rods at the crank. In my model, the bearings are rather large ball type. The rods look to have maybe bronze bush's in them and maybe yours are loose to the point of shaking the ball bearings out. Could be an out of balance flywheel. Or the mechanical flyweight mechanism is throwing the balance factor out. Look behind the cover the unloader is mounted on to see the flyweight assembly. One or more missing weight will throw balance out.
BTW, I would bet the insides are filthy with gunk/sludge. Mine sure was. As I didn't remove it from the tank it was not an easy job to get the sludge out.
 
I pulled the cover on the centrifugal unloader and as I suspected it has been disabled and the weights have been removed. I'll have to come up with a solution for an unloader but I wan't to get to the bottom of this wobble issue first.

With the cover off I could see one of the crank bearings and also grab the crank to see if there was any "play". As near as I can tell there is no play in the crank and the bearing on the unloader end looked absolutely fine.

The flywheel isn't bent or crooked as far as my eye can tell, I am really not sure of how to go about a more definitive test of the flywheel straightness and balance. If anyone is aware of a documented method please pass on the information.

If the issue were being caused by the flywheel wouldn't you think the wobble would occur regardless of the tank PSI? Again, the fact that the wobble doesn't start until the tank pressure builds makes me think its got to be either the crank bearings or the compressor and or motor base mounts.
 
Take off the housing behind that cover to get a better view of the crank and rod caps from that end. I had to make a gasket to replace the original, it was not a hard job with a small ball peen hammer.
Should be a 4 bolt access plate on either side to gain entre to the rods for further evaluation. Clean out any sludge you find, mine had such a thick sludge The oil may have not been changed EVER! Check for looseness of the rods on the crank. I would also see if the piston/s are loose on their end of the rods. Mine is a two cylinder model, not sure if your is also but ours look the same from your photo. I wonder if the poppet valves are failing at 60 psi and causing the problem. See if the pulley side bearing is in good condition while you have the access panel open.
If the flywheel doesn't wobble when turning by hand, use a DI to check it, I can't figure out why it would above a certain psi. Back pressure might do it but I'd think you'd find some looseness in related parts/assemblies as a cause.
My MK cylinders still have hone markings and pumps very well. Had to make a gasket for that too. If you can find the problem and fix it your MK should be good to go for another 50 years. They were that well built.
Again PM me for my copy of a blow up of the MK if you would like to see it's insides.
PS: The electric solenoid valve I have installed in place of the mechanical unloader is working just fine.
 
I have removed the side access cover previously. I didn't find an excessive amount of sludge, I ended up wiping out what I could and have since changed the oil twice after running a while between oil changes. When I had the cover off I wasn't aware of the wobble so I didn't inspect it as closely as I otherwise would have but I definitely checked the rods and crank for any amount of noticeable play and everything checked out fine. It is easy enough to remove the side cover so I will take your advice and pull it again and do a more thorough inspection. I have also removed all of the valves and cleaned them and made sure they were seating reasonably well, the only issue I found with the valves apart from being very dirty was with the exhaust valve spring on the smaller of the two cylinders, the spring was fairly weak and bent slightly off center. I did my best to get it straightened and put it back in place. I'd like to find a generic spring that would work for the valves, I checked with collins equipment and the parts are extremely expensive and I didn't find anything unique about the springs that warranted spending that kind of cash. If memory serves me correctly individual valve parts cannot be purchased, you have to buy a entire valve kit which is about $140.

I think I am going to go ahead and pull the flywheel and make sure it is properly seated and see if I can get my hands on a dial indicator to make sure it is true.

If you have a MK series they are the same with the only differences between models being the bore and stroke. According to collins equipment all MK series pumps use the same parts except for piston and rings.
 
In my search I did not find Collins. Glad I didn't because I might have purchased parts from them instead of making modifications which have worked OK for me.
The larger cylinder pumps into the smaller and it into the tank for the two stage operation. If you find no other reason for the wobble look at the valves to see if a bleed back may be causing the issue. Not sure how that would work to wobble the pulley/flywheel. Could it be the motor pulley causing this feat? Or worn belts? Cracked mounting plate? Is the wobble axle or rotational? I certainly am not an expert but it is strange for the effect to show up at or over a rpm instead of always. Hope you find the problem and lets us whom may be watching this thread know what ya find. I am surely interested.
 
I tinkered with it a little today and found something I hadn't realized. My thought was that the pulleys might not be aligned between the pump and the motor. In the process of checking the alignment I realized that the wobble is coming from the motor, the compressor does not appear to be playing a major role. The entire shaft pulley and windings of the motor are walking back and fourth toward parallel to the motor shaft. I found that I am able to move the motor pulley and shaft about an 1/8" by pulling or pushig on it. I am not sure of this amount of play is normal and can be minimized with by alignment and belt tension, I'd appreciate any advice on the topic.
 
While I'm no expert on motors, or much else, I am sure 1/8" play is too much. Whether or not this is the ghost you are looking is debatable but may well be a contribution. Might it be "walking" @ 60 psi and causing a vibration seen in the pump pulley? Very possible. That, with belt/s wear, could appear to wobble pulleys. Maybe a wear washer in the motor has worn out or off. I'd think such a condition would cause an abnormality in the motor function as the armature walks around. Certainly should not be a thing the motor was built to do.
Perhaps the motor bearings are bad and this shows up in the pump somehow. Or manifests itself in pump pulley wobble. If the motor is a brush type, the commutator may be damaged form the walking. Or the windings could become damaged. I'm not sure if a bad area of winding would cause an interrupted RPM "flow" or just burn out the motor. I would check that motor for the reason of the sloppy shaft.
Hope you've found the culprit. Good hunting.
 
I read a few threads about these old motors and a certain amount of play from front to back but they will typically find "electrical center" and run smooth thereafter.

I still believe the alignment of the pulleys and the the belt tension could have something to do with whats going on. Keep in mind, I removed the motor and pump from an old tank and put it on a different tank with a set of new belts. I have done my best to get everything aligned using straight edges and tape measures but I am certain it isn't dead on. How tight should the belts be? I was shooting for 1/2" to 3/4" deflection but that is pretty hard to achieve with the shear size of the motor and pump. I have notice that more tension on the belt causes the motor to start walking back an fourth regardless of the tank pressure.
 
Yes the armature should have some end play but your end play sounds excessive to me. And tension will affect the moving parts moving around.
I am reminded of the time I installed a belt drive on my '52 Harley. It was in a custom frame and the belt was too tight in the most forward (read slack here) adjustment position. When the throttle was at idle the belt would track in towards the engine but when power applied the belt would track away from the engine. I think the same principal is happening here. I don't know if an idle pulley for taking up slack would stop this from happening. If the pump shaft and motor shaft are not aligned in both planes it could be the ghost of your dilemma.
At lease I feel we are getting closer. Compile all the factors and the combination may well be your problem. Above 60# and all these factors could be coming into play to create this "wobble"

Edit: after posting this I found your picture and my opinion is it is a simple thing to produce. A length of rod to make "slugs" for the # of pieces you are to make and suitable balls & set screws. If your plastic supplier does not have a .201 diameter you may need to turn a .250" rod to size. Do so one slug at a time & cut off. That is so you don't have too much hang out from the work holder.
Make the ID threads, drill & tap the 1/4" hole, deburr and insert the slug. I don't think you want the slug to drop out of the hole so it may need to sized to be pushed in. Or maybe a bolt can be installed and the slug forced in to swaged in place. This you may have to experiment with to find the best way. Nylon might be the best material but the plastic supplier will know.
 
I toyed around with the alignment and belt tension and now it is getting up to 90 PSI before the problem starts, things are moving in the right direction. I think I am going to have to create a jack screw adjusting plate to get the belt tension where it needs to be.
 








 
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