tool blade speed

[CarvedTones]

Can someone explain to me the science (assuming there is some) behind this? I just replaced a 3450 RPM motor on my little TS with a 1725 and getting a larger sheave is on my todo list. The motor had a sheave that was larger than the one I had on the old motor, but not large enough to make up the difference. I hooked it up mostly just to make sure I had a workable set up (the motor is adapted for this use). Curiosity got the better of me and I raised the blade and made a couple of cuts. It cuts slower, but substantially quieter. The saw is an old Crafstman 9" benchtop and it came to me with a 1/2 HP motor. That is a little weak for some applications and it bogs a bit sometimes. But at the lower speed it handled a cut that was problematic before. It's hard to describe exactly, but slowing the feed rate with the faster blade isn't the same.

Along the same lines, I am close to firing up an old 4" jointer with an almost identical motor that I also need another sheave for. Reading up on it, the suggested RPM of the cutter is around 4K. I noticed the newer benchtops claim about 10K. I think the sheaves on these motors combined with the small tool sheaves is giving me around 3K.

 

[Mike Davis]

Sounds like the newer motor has more torque.

What is the HP rating on the new motor?

Is it capacitor start and not the old one?

 

[CarvedTones]

They are both 1/2 HP cap start. The new one is a washer motor, which I had read has more torque, but I don't quite understand that when they are rated the same power and run at the same speed. I probably get a few eye rolls for that, but you can pick up used ones in very good condition for little or nothing and there are "cheater cord" (what repairmen use to test a motor) diagrams easily found by Googling (but a GFI circuit and a well insulated switch are still a very good idea for testing).

 

[BumoutBob]

My Rockwell lathe had a 1/2 HP Sears on it and it wasn't very strong. We need an electrical engineer to help us with this discussion. One that knows motors.

One thing about motors is the flywheel effect. Some of the lightweight motors have just barely enough field to get the job done. They're so light there's no inertia to help spin the load. I have a 1940's 1/2hp 1725 motor on my 6" jointer that is very heavy and runs the cutters at 3450 with a large motor sheive. The spinning weight provides some torque to the load. Someone out there may be able to tell us if there is a optiminal sheive size for a given speed/HP/belt.

 

[pcooper]

My only input here is you're going half as fast/twice as slow. I did a replacement on a motor like that one time to get me by with a job, happened to have the slower motor laying around already was the reason, and found that the slow motor just didn't have the gumption of the faster one, rated the same HP, and didn't last long due to the lack of speed, and it burdened too much doing what I needed done. I'd go back to the faster motor, and not try to speed up the blade with pulley changes, you'll loose torque in doing so, and find the slow motor won't hold up. That's IMHO only you know. :nah:

 

[CarvedTones]

Phillip,

I think you missed the part about me having a larger sheave, just not large enough to make up the total difference. At the blade, I am looking at about 3000 now versus about 4000 before.

Bob,

I hear you about the flywheel effect. The outer diameter of the spinning part of the motor was smaller with the old motor. Another effect that occurs to me is that the slower motor, by virtue of having a larger sheave, is in contact with with more of the belt. That, coupled with the slower speed, virtually eliminates slippage.

 

[CarvedTones]

A quick reply to myself as I noticed I was probably a bit too terse in my reply to Phillip...

I am not discounting your input out of hand. I think it is probably a case by case thing; in my case what I noticed is that the slower motor seems stronger. I probably should have been more explicit about the "cut that was problematic before". It was some white oak that was bogging if I wasn't careful about the feed rate with the faster motor. I felt like I could cut it faster with the slower speed because I did not have to back off on the feed rate.

 

[BumoutBob]

In searching around the net I found this site
http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm
which provides some motor formulas. The one on torque is the most interesting, it says torque has an inverse RPM component. For the same HP the lower RPM motor has more torque. Since you have to use a two to one pulley ratio to get the saw blade correct you may loose the torque advantage. But, you are using 1.5 to 1 ratio so you probably gained some torque.

In the long run----get a bigger motor. Interesting to go to the Baldor website www.baldor.com and study all the motors they sell. Amazon used to sell Baldor motors but they don't seem to be available there now. Their farm duty motors are good for shop use.

 

[cptully]

Andy,

Although I cannot give exact answers, based on what I remember from my college courses, the lower speed motor with the same HP rating will have more torque, because the speed versus torque relationship is inverse (all things other than HP rating being equal). More torque equals lower speed. Think of putting your car/truck into low gear when climbing a mountain or towing a a heavy load.

The larger sheave on the new motor also helps with the torque situation, both because of the larger belt contact area and because of the increased moment of inertia.

As for better cutting with a lower blade speed, try cutting a board with a hand saw: Take both fast and slow strokes and look at the results. The faster strokes will generally cut shallower than the slow even strokes. At a lower blade speed, the saw teeth have more time in contact with the wood.

Chris

 

[CarvedTones]

Bob,

Yeah, I agree that a bigger motor (as in more than 1/2 HP) is a good idea. It just wasn't easy to justify on the spur of the moment. Two motors in great condition for a whole lot less was easier. The 4" jointer called for a 1/2 HP maximum, so one of the motors is plenty adequate for it. And the TS hasn't been unusable with the low power, just had a rough time with the WO but managed.

Chris,

I understand about speed and torque, but I was reading about these motors having more torque at the same speed. I think I understand it now after more reflection; 2 motors might be able to provide the same power at the same speed after having spun up, but they may have different characteristics powering up. One may have troubel powering through initial resistance and the other may not.

 

[Charles M]

The larger sheave on the new motor also helps with the torque situation, both because of the larger belt contact area and because of the increased moment of inertia.

True that the greater belt contact area reduces slippage but the larger pulley on the motor decreases the torque.

 

[CarvedTones]

It decreases the torque compared to the same motor with a smaller belt. I get that. Let me try to explain what I don't get...
A 1/2 HP motor spinning internally at 3450 using a sheave with a circumference (not diameter; trying to make the math easy) of 4" will pull on a belt at the same speed as a motor spinning at 1725 using a sheave with a circumference of 8", correct? Is there a reason that one would usually have more torque than the other or is it just dependent on the specific motors being used?

I still have my original question about blade speed also. What is the reason to prefer higher speeds with less torque? I tried the jointer I am rehabbing at a "low" speed of maybe 3000 or so and edging a board by 1/32" it really wasn't noticable. If I tried to face a 4.5" (full width of this little guy) hickory it would probably be a different story.

 

[pcooper]

"I think you missed the part about me having a larger sheave, just not large enough to make up the total difference."

Andy, there's a lot of confusion concerning torque, and here is how i approach this. The driven sheave and the drive sheave have different properties concerning torque. If you increase the size of the driven sheave, you slow down the shaft but increase the torque applied to it, and if you increase the sheave size on the drive shaft, you lower the applied torque from the shaft to the sheave, but increase the speed of the output. They are directly proportional. The contact area on the belt helps with slip ratio, but not torque. I wish I had an example from the web or elsewhere that I could use, but I spent a lot of time on this in college, and unfortunately all my examples are now in my head or put onto a white board in classes I teach. Inertia applies to it as you stated true enough, but only if there is sufficient mass, such as a heavy flywheel, and something like a table saw blade isn't enough mass to matter. We're getting so deep into physics here...:confused_ I think I understand your thinking, but you may just have to try to see what happens. There's no better teacher than experimenting and trying. :widea:

 

[Mike Davis]

Torque = force x radius = lb x ft = T
Speed = rpm = N
Constant = 5252 = C
HP = T x N / C


torque x speed(rpm) / 5252 = HP
So a motor at the same hp but lower speed has to have more torque.





http://www.reliance.com/mtr/mtrthrmn.htm


Section 3: Horsepower Basics

In 18th century England, coal was feeding the industrial revolution and Thomas Newcomen invented a steam driven engine that was used to pump water from coal mines. It was a Scott however, by the name of James Watt, who in 1769 improved the steam engine making it truly workable and practical. In his attempt to sell his new steam engines, the first question coal mine owners asked was "can your engine out work one of my horses?" Watt didn't know since he didn't know how much work a horse could do. To find out, Watt and his partner bought a few average size horses and measured their work. They found that the average horse worked at the rate of 22,000 foot pounds per minute. Watt decided, for some unknown reason, to add 50% to this figure and rate the average horse at 33,000 foot pounds per minute.

What's important is that there is now a system in place for measuring the rate of doing work. And there is a unit of power, horsepower.

If steam engines had been developed some place else in the world, where the horse was not the beast of burden, we might be rating motors in oxen power or camel power. Today, motors are also rated in Watts output.


hp = lb x fpm / 33,000
hp = ft-lb x rpm / 5,252
kW = hp x 0.7457
hpMetric = hp x 1.0138
Horsepower as defined by Watt, is the same for AC and DC motors, gasoline engines, dog sleds, etc.

Horsepower and Electric Motors

Torque = force x radius = lb x ft = T
Speed = rpm = N
Constant = 5252 = C
HP = T x N / C
Torque and DC Motors

T = k Ia
At overload, torque increases at some rate less than the increase in current due to saturation

D2 L and Torque

258AT = 324 D2 L
259AT = 378 D2 L
With the same frame diameter, the 259AT has 17% more D2 L and thus 17% more and 17% more Torque. Motor torque increases with an increase in iron and copper, combined with current. It can then be said that it takes iron and copper to produce torque and torque makes products. Or to put it another way, what you purchase to make product is TORQUE and that is IRON and COPPER. The rate of doing work is power and HORSEPOWER is a unit of power.

 

[Mike Davis]

Second question.

Why is higher speed preferred? (lower torque is not preferred but is a by-product of higher speed at same HP)

It comes down to surface feet per minute of the blade at the rim. 9000 SFM is recommended for hardwoods. That is around 3450 rpm for a 10 inch blade. This gives the smoothest cut without burning the wood. Softer woods can be cut at a higher speed and harder woods require a slower speed.

Most table saws are not variable speed so a compromise is set.

 

[CarvedTones]

Wow! That is lot of good information. One thing jumps right out at me:

Torque = force x radius = lb x ft = T
​

and I had posted:

a circumference (not diameter; trying to make the math easy)

​

It appears the math can't be as easy as I want.

Horses played another part in some historical unit trivia (though it is debated and might be folklore). The width of rails on the railroad is roughly twice the width of what I am making of myself by continuing to question this...