935 centrifugal switch

[petertha]

Well, I don’t know if I bought me some time or issue solved. Let’s hope for the latter. I figured there was nothing to lose to have a look-see of the centrifugal switch. I removed the motor end bell, plastic fan & this exposes the centrifugal switch & underneath that, the spring washer points plate. I noticed a glop of Loctite on the screw holding the CS onto the motor shaft so I first scribed a line on the shaft so I could replicate this vertical position because it’s very likely factory set against spring washer tension. Now was it set ‘correct’ to begin with & I mindlessly replicated I can’t say.

Once out, the CS action seemed relatively smooth. I couldn’t see any holdups or burs. It’s not exactly a Swiss watch, just stamped metal parts & press pins. But I couldn’t feel any noticeable snags or lockups.

Next I removed the spring washer points plate out. For the most part looked pristine. But I noticed just a hint charcoal looking burn & teeny frags like pitting on maybe 40% of the disc. I figured what the heck, clean it up with an Emory board. They looked flatter & shinier when I was done but it was more just to clean them up while I was there.

I put all the CS parts back the way they were. One thing I didn’t pay attention to was where the cooling fan was placed vertically when I removed it. Actually it was removed by the tech once before on episode-1 so likely not factory position anyway. It is retained on the shaft by a Philips M5 screw & nut which is cumbersome to remove. If you come at it with a long screwdriver it is possible to kind of slide the unit down trying to torque it. Hmm.. I wonder if maybe far enough down to conflict with the CS? So I swapped in a socket head cap screw so I could engage more positively & kept it high on the shaft with max clearance to CS. Buttoned everything up & figured WTF, plug it in & see if it cogs. Surprise. Smooth running motor. Aaaah. Tried it a dozen times in & out of gear, forward, reverse. Its like the problem never existed.

So I don’t know what to say right now. Maybe the points, maybe the fan position was the underlying problem. Or maybe this just bought me another 200 starts & the cogging will start again. Maybe I am just on the bubble of some cumulative effect of electrical issues stacking up the wrong way like +10% motor draw + 15% cap tolerance issue + + 5% N-Am line voltage issue + ??? maybe it might come to haunt me again. So I’m going to try & run the crap out of it now.

Thanks for your help & suggestions thus far. TBC.

 

[hman]

I have a Jet 6" bench grinder that was refusing to start. Don't recall how long it's been ... possibly since my move from Oregon. When I tried to switch it on, it would just buzz. If I pre-spun the wheels before turning on the switch, it would start (most of the time). Happy to see that, as it demonstrated that the run winding was OK. Today the tuit became round enough for a repair.

Checked the capacitor - OK. Opened the end bell of the motor, removed the centrifugal switch operator and switch plate, checked switch contacts - OK. Checked the centrifugal linkage and spring - OK. Ohmed out all the windings - OK. Head scratching time! So I started to reassemble the motor and finally noticed something. The centrifugal switch, held onto the shaft by a setscrew, was too far out and would not close the contacts with the motor stationary. AHA!!!! I guess the switch slipped at some unknown time, possibly by the grinder being jostled during the move, or possibly it just slowly worked itself out of position.

Re-installed the switch a bit further in, closed up the motor, and voila! Problem solved.

 

[qualitymachinetools]

Hey Petertha, Im not sure who you are, let me know what name it was bought under. Its the point contact sticking that took out that capacitor, we had them update that contact for us and so far it hasnt happened again with the better part. We had a few of these over the past year. It was a simple as the contact sticking on when the motor was running, so not letting the start cap out of the circuit, then burning it right up. I checked the motor serial # with the factory and they were not finding it for some reason, email us at parts@precisionmatthews.com and let us know what name it was bought under and address, they will send you one at no charge.

 

[wrmiller]

Gotta love PM's customer service!

 

[petertha]

... let me know what name it was bought under....

Wow, that is very gracious offer and thanks so much for your technical insight on this issue. I will for sure forward you the SR# and vendor information.

 

[qualitymachinetools]

No problem, not sure why it is not coming up by that serial number, is that the original motor? We will figure it out, just send your name and approximate date we shipped it to you and I can look it up.

 

[petertha]

Some more to the story. I've been happily milling away for several months back & the dreaded cogging sound started again. If you never experienced count yourself lucky. But if you haven't, it resembles a loud mechanical noise like a gear case with some loose extra bolts. So by now I know the drill. Shimmy up the ladder, remove the motor grill, examine the plastic fan to see if it had migrated down to interfere with the centrifugal switch. Last time I mounted it flush & now it was maybe a smidge lower, but not by much. I've Loctited the retention bolt/nut but its basically pressure clamping the boss portion of the blade & not the most robust design. When I remove the fan I can see that the CS has not migrated up because I can still see my witness scribe line on the shaft from before. Remove the fan altogether, flip the switch, motor starts & purrs like a kitten.

So now my theory is this is just a bad design or they switched fan blades or something goofy like that. The underside of the fan is kind of a dome shape so its obviously quite close to potentially interfering with the CS counterweight action. The problem is there is no reference or mechanical stop to prevent this interference from happening. So I turned a nylon spacer disc, 0.75" ID by ~ 1.0" OD x 0.30" thickness. It's a little fat but I'm going to try it this way. Now fan sits a little above the end of the motor shaft but still lots of ceiling room to the motor grill plate.

I set the disc on the CS hub. Now there is no way for the fan to interfere anymore. If it drifts down or get inadvertently gets pushed down, it will come too rest on the spacer but still leave room for the CS arms to swing up. Well about 100 starts since its been installed, fingers crossed and no sacrificial incense to the capacitor gods.

(If I didn't mention, its not a PM but its a very similar 935VS model).