Friends:
Yes, I am a bit, er, thrifty and cannot stand to see good, fully working objects go unused. Yes, I will stipulate 6that it would be easier to just buy a bench grinder and be done, but that is not the course on which I have embarked. So, heading off in advance all the good advice that might relate to bench grinders, here is my project and my problem:
I already have a couple of rehabilitated hand-powered grinders that work well for edged tools, and I have a WEN wet wheel for other kinds of smaller edges like scissors. However, there are times when you need a good buffing wheel, or possibly a hard grinder for repairs. So, I bought LV double mandrel and plan to build a new sharpening station to fit all of the above.
Here is the problem: I have some very sound orphan motors. They are pretty hefty. Two are from washing machines, and one used to drive the squirrel cage air mover in our late lamented HVAC system. The air mover motor has a good base already attached. I am thinking of using it to power the mandrel. No question it has way more than enough power. However, there is no nameplate that I can find, so I don’t know the rotational speed. If I knew the speed, I could adjust the mandrel speed down to what I want by making a simple belt driven reduction train. But I don’t know the starting speed.
Any thoughts on how to approach this?
Joe
Replies
If the motors don't have brushes they are probably either 1750 rpm or 3600 rpm although a multiple speed motor might have an additional speed.
The difference in speed betweeen 1750 and 3600 is easy to spot by just looking at the spinning shaft. Take a look at a spinning motor that you know is 3600 or 1750 rpm and compare the it to the speed of the mystery motors, it should be fairly easy to tell if you have 1750 or 3600 rpm.
John White, Shop Manager, Fine Woodworking Magazine
Good idea.
What if the motor DOES have brushes?
Joe
In a thread on router speed lwilliams provided this link:
http://www.web-tronics.com/digphottac.html
THis would give you an actual measurement of the RPM and it seems it would be useful after your set up your mandrels, etc.
I didn't know those phototachometers existed, and would not have guessed at the low price. Tanks for the information.
Joe
If the motor has brushes it will not settle down to a steady speed that is some multiple of the power supply's frequency. Typically, brushed motors, called universal motors, run at very high speeds, ten to twenty thousnd rpm or more.
Brushed motors are common in portable tools like circular saws and routers because they are light in weight compared to their power, but are almost never used in fixed larger applications such as appliances, so it is unlikely that the motors you described are brush type.
John W.
Oho. Right you are. These are not light, and not likely universal.
Joe
Some of the older fixed tools used the repulsion induction motors. The RI motor is brushed and is very good for hard starting situations, i.e. air compressors. As far as I know, they are rated for only 1725 or 3450 rpms.
Dat is correct-just for interest here are two repulsion motors with brushes. The silver one should be familiar to most whilst the red one is A British make 1/2 horse which I use to drive my buffing wheels-the interesting thing about this is that it is reversed by rotating the plate on which the brushes are mounted through 90 degrees- an electrician lost a bet with me on that fact.Philip Marcou
I have an old RP motor that is 1/3 HP. A motor mechanic once said that size was very rare. That was fifty years ago so I am sure it is more rare now. It hasn't been used in forty years and since it sat out in the weather for a year or two, it might not be any good now. Yes, now that you mention it, I do remember turning that plate to reverse the direction.
You are correct, there are motors other than universal style that use brushes, but repulsion/induction motors went out of style quite a while ago in the U.S., I haven't seen one on small equipment made after the mid 1950's.
I'm not familiar enough with them to know what their typical operating speeds were but being they were induction motors when running they probably were 1725 and 3450 rpm also.
John W.
I think the higher speed induction motor is 3450 rpm, isn't it?
BruceT
You are correct, nominal speed on capacitor start induction motors is listed as 1725 or 3450 rpm.
John W.
"The difference in speed betweeen 1750 and 3600 is easy to spot by just looking at the spinning shaft."Your post reminds me of some wonderful high-speed movie photography done in the 1950's using a strobe light. The cinematographer set his strobe light to flash at exactly the frequency of the subjects he was filming so that they seemed not to be moving. Water droplets streaming from a faucet appeared to be suspended in mid air and, best of all, a table saw blade was filmed from the side so that you could clearly read "Craftsman" on it - then a piece of wood was run through the apparently motionless blade.
BruceT
I just went back to your original message and noticed that you said one of the motors was a squirrel cage type, I missed that the first time I read the question. That style motor usually have a running speed of 1000 to around 1500 rpm although there are a few that run at 3000 rpm.
John W.
That makes sense, John. For years I had a felt buffing pad directly attached to the shaft, and it did not seem inordinately fast. I don't want high speed anyway, so with a reasonably slow motor and a small pully on the shaft I should be OK, wouldn't you think?
Joe
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