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.
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.
Let's see, RPM X Torque = Horsepower.
Thus: No-load RPM X Locked-rotor Torque = Sears Horsepower
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
Source (again) RepairFAQ.org's humour page.
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
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.
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.
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.
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.
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