Horse power, horse hockey

I know it's a commonplace around here, but I just have to vent.
I would like to meet the marketing people who expect me to believe that a 16-gallon ShopVac is 4 times more powerful than a $500 contractor's saw, which also is supposedly just a shade over half as powerful as my $50 skilsaw. Who the hell makes this stuff up?
That's all. Thanks for letting me get that off my chest.
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You are quite welcome.
Have you been reading any Sears advertisements by chance?
RonB

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RonB wrote:

My God! How did you know?
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Sears has had more ways to sidestep specifications and other features than any distributor I can recall. Remember 'starting horsepower'? That is probably what started their credibility gap with us older farts. You never know what they are talking about.
RonB
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Let's see, RPM X Torque = Horsepower.
Thus: No-load RPM X Locked-rotor Torque = Sears Horsepower
Notes:
1. testing done at 177V DC, equal to peak of 120V AC (AC-DC motors).
2. Sears Horsepower: How "hoarse" you get trying to talk over one of their shop-vacs while it's on.
Or maybe it's got something to do with vacuuming performance out at the stables.
Source (again) RepairFAQ.org's humour page. http://repairfaq.org/sam/humor.htm
Puckdropper
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Wise is the man who attempts to answer his question before asking it.

To email me directly, send a message to puckdropper (at) fastmail.fm
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I'm going to take the motor off of my shopvac and put it on my tablesaw. Not only is it more powerful, it's smaller! Technology is wonderful.
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I might put mine on the wife's blazer *g*
Mac
https://home.comcast.net/~mac.davis https://home.comcast.net/~mac.davis/wood_stuff.htm
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mac davis wrote:

You'd spend all your savings on it buying extension cords:-) Joe
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Good point, Joe... I don't think our 400 watt invertor would run it.. lol Mac
https://home.comcast.net/~mac.davis https://home.comcast.net/~mac.davis/wood_stuff.htm
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Since HP is the result of a calculation that includes RPM, bizarre ratings can occur. http://www.reliance.com/mtr/flaclcmn.htm Good site for formulas.
Dave
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boorite wrote:

If you're measuring electrical input power, then I've got a 4 horsepower kitchen toaster, and that doesn't even move. If you allow electricity wasted as heat into the calculation, then you can get some perverse measurements. As vacuums move a lot of air that they also use for cooling, they're usually designed to be woefully inefficient in the search for cheap power. It might even _be_ mechanically quite powerful - some of them are, despite their small size and weight.
My table saw is not only rated at 3HP, that's real 3HP at almost any speed, without risk of burning out.
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Me too. Perhaps you should understand more about horsepower ratings.
The purveyors of this stuff rate motorins things like the Shop Vac as the product of the maximum amperage times the maximum voltage. And, they don't worry about such niceties as temperature rise.
However, motors for machinery such as table saws are rated at the average power (which is 1/2 of what the above calculations show) and for an intermittent load. This load results in a temperature rise of 60F above ambient temperature. Thus, machinery motors are conservatively rated whereas those other are wildly optimisticall rated.
Jim
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So, looking at realistic HP ratings (not shop vac sensationalized values), my table saw is a 230 volt, 3hp motor. My router is a 120v 3hp motor. TS spins about 3 to 4 000 RPM, router can go up to 22K. I still find it difficult to comprehend that the two motors are rated the same. (And I trust Porter Cable!). Does the output speed make the router rate as high as the TS? Obviously it is not turning as heavy a cutter as the TS. Marc
Jim wrote:

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You shouldn't. On a 120 volt, 15 amp circuit, even 2.5 horsepower is impossible.
I (And I trust Porter Cable!).
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From what Jim said, if I understood it, the table saw could actually max at maybe 6 hp...

<snip>>
Mac
https://home.comcast.net/~mac.davis https://home.comcast.net/~mac.davis/wood_stuff.htm
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hello,

formula: HP = T * RPM / 5252 (T in lb/ft). Thus T = (HP*5252)/RPM
Let us assume that the peak HP is at 4000 RPM (induction engine), this means that to generate 3 HP, the ST will need around 4 lb /ft of torque. with a 10 " blade (5" radius), this mesures to 9.5 lbs at the teeth...

same calculation: .7 lbs /ft of torque on the router, with a 1/2 radius bit, this is 17 lbs of pressure at the teeth...

yep, directly proportional... note, it is easier to make a fast spining engine have high power than a slow spining engine as less power torque needs to be generated...

the overall weight of the cutter is negligeable compared with the weight distribution (ie: how far from the axis is the weight).
note, 1 HP = 745 W and 1 W = 1 V*A so if your tool is rated to draw up to 15A at 120V = 1800W = 2.4HP, you know, that, because of various losts in heat, friction and the fact that this is a maximum that probably never gets reached, the tool is capable of at max 2.4HP, and likely to be much closer to 1/2 to 3/4 of that value.
cyrille
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