Rebuilding a Dental Air Compressor Head

[mechanicalman]

So here goes my first post.

I was looking for aftercoolers and air dryers on craigslist when I came across a dentist selling his old Air Techniques Airstar 5 compress with one bad head for the steep sum of $150.

After reading this thread and not finding a good reason to risk it. I brought it home and set about to fix the bad head. One head ran like a champ, so I had something to compare the bad one too. Sadly, there's not much information out there about these compressors as far as service and parts (the parts I did find are incredibly expensive!).

The bad head had a hard time starting, but runs well if I take the fan cover off and help it along with my hand. Once it's going, it goes. First thing, I checked out the start capacitor which was located in the control box along with the breaker switch and run capacitor. It checked out on the multimeter (it has a capacitor function that measures capacitance), but I double checked it by switching capacitors between heads. This didn't help the bad head, so I figure I'm left with a bad centrifugal switch or a burned up starter winding (this is the single-phase electric motor rebuild I've ever attempted).

So now I'm taking the motor apart, but can't seem to figure out how I'm going to get the pump body off the motor. The heads, and valve bodies came off easily (all the o-rings looked like they'd have to be replaced), but I stalled trying to pull the crankshaft off the motor shaft. As you can see from the pictures, I broke an M10 puller bolt off in the center hole trying to remove it with a homemade puller I made from a piece of H-beam ('wide flange beam' for the engineers) I set across the front of the pump body.

I haven't removed the motor thru bolts because they can't be taken out with the pump body on, so I started soaking the crankshaft/motor shaft fitting in PB blaster in preparation for my next attempt at pulling the crank off.

Before I continue though, I wanted to poll you guys for how best to proceed. I'm a little worried that if I get the crank off (which seems to be a two piece design based on the fact that the piston rods are a one-piece design) I won't be able to press it back on to the correct depth/height if there's no positive stop to keep me from pressing it too far; this whole thing reeks of high-tolerance measurements.

An alternative I'm considering is cutting the thru bolts and drilling out the threads the far end. Then making my own thru bolts from a piece of round-stock and threading both ends so I can use nuts on both ends.

So... should I keep trying to pull the crankshaft off, or should I go ahead and start cutting bolts? I'm afraid I won't be able to put the crank on properly if I get it off, but I'm also afraid that I won't be able to take the motor apart to get at the centrifugal switch with just the thru bolts off and the pump body still on.

annotated album on imgur

 

[mechanicalman]

And here's another album with imageshack since I know everyone wants to see pictures directly in the thread.

 

[mechanicalman]

Got the crankshaft off the motor shaft after soaking it in PB Blaster for a few days and making another go with my homemade puller. Once the crankshaft was off I was able to make pretty rapid progress taking the rest of the motor apart. After getting the rotor out I was able to see the centrifugal switch was wobbling on the shaft due to a failed rivet that I found sitting in the bottom of the motor housing. The backplate switch had a lot of dust that I presume to be graphite powder from the contractors that the switch mechanism rides on.

The bearings all seemed to be in great shape, but I pulled them off the shaft and ordered replacements since I never want to take this thing apart again. While looking for a replacement centrifugal switch and parts, I came across this. Seems like a great way to eliminate a mechanical failure point, but I'm not really clear how I'm supposed to wire it for a 220v motor. I'll make a separate post about this. This one is going to be about taking the motor apart and confirming what was wrong.

aaaaand, imageshack tricked me into deleting all the pictures from this album, so here they all are again, plus ones illustrating what I did in this post.

 

[mechanicalman]

So I'm trying to figure out how to wire the ISOVOLT DMS03U starter to my motor, but I'm having trouble figuring out where to connect the N terminal.

The terminal box on top of the motor has four wires coming into it from the motor housing/stator. One of them is the lead going to the centrifugal switch. The other three go to the stator windings. I concluded this based on very low resistance readings relative to each other. There's also one going from the centrifugal switch area to the start winding I assume. I erroneously noted in a the photos of the previous post that this wire goes to terminal 7 on the terminal block.

Looking for more info, I Googled the motor model # on the tag, LS90L, and it led me to the Leroy-Somer website and a datasheet for a line of three-phase motors that look exactly like mine. Then I find this schematic for running a three phase motor on single phase 220v power.

Could it be that Air Techniques ordered a three-phase motor from Leroy-Somer setup to run on single-phase power? Even if the motor is three-phase, I can still use the ISOVOLT starter in this configuration.

I guess I need help figuring out for sure if this is a three-phase motor setup for single-phase, or if it's a single-phase motor by design; in which case, how do I figure out where to connect the N terminal?

 

[Dieselbutterfly]

is it possible this thing could use a hard start kit like the type you would put on a household central a/c unit

 

[mechanicalman]

It looks like my motor actually jives with another schematic that I very stupidly didn't include when I posted a screen cap of the ISOVOLT instruction sheet. I believe my motor works with the top schematic. Previously I only posted the bottom two because I thought the top one was for dual voltage motors. In this case it's pretty clear to me where the N and L terminals should go.

I'll post a schematic of how I think this will work later, but right now all I have is something I scribbled on a scrap of paper I had in my pocket.

 

[mechanicalman]

Success! I used that top schematic as my template for how the motor was wired and everything made sense very quickly. Here's how I determined the motor was wired. That first schematic I labeled was completely wrong.

Here's that schematic from ISOVOLT again (we're using the top one):


And here's how I wired it. The only 14 awg wire I had was green so that's what I used.

I picked the common lead at the main switch terminal on the front panel and connected that to the N terminal (this is switched voltage). The L terminal is connected to the line voltage at the top of the run capacitor in the foreground. The C terminal is connected to the where the blue wire was on the start capacitor (obscured behind the run capacitor), and the A terminal is connected to the other side of the run capacitor.

I put it back together and the whole thing started up strong as it ever was!
Here's some part numbers for what I've used so far.
6205 Bearing
6306 Bearing
DMS03U (the isovolt)

Taking the compressor head apart also revealed a lot of toasted o-rings. out of the four heads, three of them had the main o-ring blown out just like this one where the groove doesn't support the back of the o-ring.

I'm waiting on viton replacements and decided against getting silicone because of the lower tensile strength despite better hi-temp tolerance.
Here are the o-ring sizes (all 1/16" cross section):
033, 027, 025, 015
You need one of each size for each side of the head.

And that's it! I guess there will be another update when I get the o-rings and I post some results for how it performs.

 

[dmgdds]

Hi mechanicalman, I just saw this thread on rebuilding an LS09L compressor head from a dental Airstar 5. Are you interested in helping me do the same with one the heads on my Airstar 5 compressor?
Thank you, dmgdds

 

[dmgdds]

Oops, meant LS90L compressor head.
The right head shakes some when it starts up and occasionally trips the breaker.

 

[Timclubb29]

I’m currently in the process of repairing a dental air compressor nearly identical to the one you rebuilt in the thread. My big question is about the crank. Was the one bolt that was hard to reach the only bolt holding it in and did the crank come out in separate pieces or together? I assume it’s a two piece crank since the rods don’t have removable caps. Any info you could share would be greatly appreciated as your thread is the only information I could really find about disassembling the compressor and motor. Thanks in advance

 

[The Cobbler]

note this thread is 7+ years old & the OP has not logged on to GJ in about 3 years