pully size for jointer


on the same rebuild of the 6" craftsman jointer - needed a pulley for the motor. original was 5", all Northern Tool had was a 4-1/2". obviously this will increase the rotational speed of the jointer head by something close to some 5% the radius and some factor of pi - i dont care about the exact math - i just know it will be a bit faster. should this be an issue or should i look further for a 5" pulley?
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With the smaller pulley you will have marginally less power and the belt will probably have to be shorter.
With that in mind, my boat anchor 6" Craftsman jointer only has a 2.5" pulley on the motor and a 2" pulley on the cutter. IMHO if you maintain that ratio approx. 4 to 5 you should be ok. If you pullies are much different than that I would say that your cutter rpm's will be way off.
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Actually the cutter head RPM will be reduced by 10% (4.5/5.0).
WW Grainger will have 5" sheaves, so should any decent hardware store.
If not, local hardware will order.
Lew
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----- Original Message -----
Newsgroups: rec.woodworking Sent: Wednesday, October 07, 2009 12:21 PM Subject: Re: pully size for jointer

Actually .... nearly twice that Need to compare circumference, not diameter. Speed will be closer to 19% slower (15.91/19.64)
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wrote:

Oops ....... brain fart My apologies, ......You're right I was thinking of area, which makes no sense at all
: )
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snipped-for-privacy@nono.com wrote:

If it's on the _motor_, going from 5 inch to 4-1/2 inch will *decrease* the rotational speed--from a safety viewpoint there should be no problem with it at all unless the smaller diameter results in the belt coming into contact with something that it shouldn't. The decrease will be about 11 percent.
That said, try Grizzly and McMaster--both list 5 inch pulleys for a reasonable price. If you absolutely positively gotta have it today Grainger has them too (4.95" anyway, which is close enough) but they want twice what McMaster charges.
Read the fine print at McMaster though, about belt types and effective radius--two pulleys with the same outside diameter can run the belt at different depths in the groove and give very different RPMs.
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snipped-for-privacy@nono.com wrote:

The factor(s) of pi cancel out so it's simply the ratio of radii. Others have noted the difference is somewhat slower on the driven side instead of faster.
I'd only comment that whatever you decide on the size, get a good quality turned pulley, not one of the cheap-ass pot-metal things on the assorted-sizes rack...
--
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On While I cannot comment on the Jointer question directly / from experience, I can offer an accounting of an experience helping an electrician pal years ago with a restraint exhaust fan problem he was having.
It seems they called him when the fan stopped working and he came, found a broken driven pully and replaced it - problem solved, fan working. He got paid and left.
He wound up calling me to take a look because the fan stopped working the next day - blowing the breaker (and the breaker he replaced) and he could not figure out why.
I climbed up on the roof to find that, fortunately, he was a less than diligent repairman and hadn't cleaned up the work space after leaving the first day. I found a bit of the original pulley laying about near the fan motor and asked if it was a piece of the old, broken pulley.
When he confirmed that suspicion, I held it up against the replacement he'd installed and determined that it was larger (or smaller) by a half-inch or so than the original.
Off to ACE Hardware and back to install the correct size pulley on the fan and the problem was finally solved.
For what it is worth, tools are designed by engineers who have done all the calculations and assembled a device with all the appropriate parts. Absent some Engineering knowledge and an intent to "tweak" your tool a bit, safe bet to to replace parts with the exact same thing they were designed with.
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Hoosierpopi wrote: ...

Well, DOH...
The story would indicate more than simply a pulley diameter change problem--the additional torque of a minor speed increase causing failure to start would only be an issue if the starting load were already very near limit of motor and circuit or the change was a very significant percentage change.
For a jointer a 5-10% speed change is minimal altho slower rpm will require a slightly slower feed rate to continue to get the same degree of smoothness in the jointed edge...
--
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It is not that important to keep everything running at a speed determined by the engineers. That speed changes when you start running stock through anyway. It really slows down when you start surfacing a 6" wide board. Granted a drastic pully diameter deviation would not be a good thing but a small jump such as the one proposed would probably help to maintain speeds.
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