From: John De Armond
Subject: Re: portable air tools
Date: Sun, 15 Aug 1999 06:58:27 EDT
Newsgroups: rec.autos.tech

william Henry Grosvenor wrote:
> 
> I'm looking for a good air tool set up.
> Can anyone recommend a good compressor and tool setup that fits in a
> truckbed and works off of an inverter?
> TIA

That's easy.  None.  Do the math.  Figure with losses an electrical
HP is worth 1000 watts.  1000 watts @ 13.8 volts is 73 amps,
assuming a 100% efficient inverter.  The 1000 watt inverter in my
catering truck draws 120 amps at full load.  It will barely start a
1/4 hp refrigerator compressor.  A compressor motor may require as
much as 6 times run current to start, even more if the  unloader
doesn't work.  We haven't even looked at alternator capacity yet.

You have several options.

the best option is what some of the tire roadside service trucks use
- an engine-driven compressor controlled with a magnetic clutch
similar to the one on an AC compressor.  These installations
typically put the tank up under the frame rails so you don't lose
any bed space.  All the air you could possibly want for air tools.

An option if you also need 120 VAC power for other loads is a truck
engine driven alternator.  Norther Hydraulics sells such alternators
rated up to 10 kw.  Requires the engine speed to be set and governed
to maintain the frequency.  A conventional electric compressor could
then be used.

the next best option is a gasoline-engine driven compressor.  Bulky,
noisy, expensive and  yet another engine to maintain.

There are fractional HP 12 volt compressors available but they
typically produce less than 1 CFM at 100 psi. (not talking about
those crappy little compressors Wal Mart sells for [eventually,
hopefully] inflating a flat.)  I have just such a critter tucked
away in my trunk for inflating tires.  It would take forever to pump
up a storage tank with enough air for even a few seconds of air tool
use.  And they cost >$200.  Check out Graingers if you're
interested.

John

From: John De Armond
Subject: Re: DC Powered Air Compressor
Date: Wed, 22 Sep 1999 22:39:08 EDT
Newsgroups: rec.outdoors.rv-travel

TROUTFISH2@webtv.net wrote:
> 
> Looking for feedback as to the purchase of a dc powered air compressor,
> suitable for inflating 60 psi on r.v. tires, or less on other vehicles
> if necessary.  I have heard that some brands don't have the "strength"
> to actually push enough air into a 28 footer grossing approx. 5500#,
> which is what I have, and it would certainly be a lot simpler than
> maneuvering same in the typical gas station parking lot.   I'm
> assuming that there's one out there on the market with a long enough
> cord on it to reach from tow vehicle cigarette lighter [or dc plug in
> r.v.] allllllllllll the way back to the tires.

If you want something that will actually inflate a tire before your
vacation is over, it will be a bit more complicated than buying a
wal-mart special "air compressor" (sic) and hopin' and prayin'. 
There are several reasons.  

The first reason is that a compressor that costs much over $30 will
lack the mass market appeal to be considered by the major marketers
(wal-mart, K-mart, Sears, etc).  That pretty much limits what
content can go into a product.  The second consideration is that
only a very limited amount of power is available from the cigarette
lighter plug.  It is generally regarded as pushing the limit to
attempt to draw more than about 8 amps from the plug.  Yep, some are
rated and fused for much more but consider the situation from the
manufacturer's perspective.  A customer might buy the thing, try to
use it in a Yugo or something, set it on fire, kill someone, sue the
mfr and win a $50 billion judgment (hey, it happens to GM every
week, it seems.)  A final consideration is that there is a limited
amount of power that can be pushed through the typically 16 ga cord
used on consumer appliances.  A higher gauge could be used but then
it would be bulky, it would strain someone's carpal tunnel syndrome,
they'd sue, etc.

There is an alternative.  The solution consists of two parts, a)
sufficient 12 volt power and b) sufficient compressor capacity.

Item B is easy.  W.W. Graingers (among others) sells a 1/10 HP 12
volt powered oilless compressor.  It draws 10 amps.  Go
here:http://www.grainger.com/cgi-bin/ngs?634540YXkk2860kk5Z679 and
look at 5Z349, center of the top row of pictures.  Cost: $157.00. 
This compressor will need either a relief valve or a pressure switch
(elsewhere in the Graingers catalog.)  This is a very nice
compressor.  I've used several in various applications.  I mount the
compressor and a relief valve in a Milwaukee circle saw metal case
along with some hose, a milton qucik coupler and some heavy-gauge
wire.  Makes a very nice package.  This compressor could also be
mounted permanently on the RV along with a small air reservoir and
enough hose to reach all around.  Graingers does not sell to
individuals.  You can either find a Graingers dealer or do what most
people I know do, make up a company name and buy with cash at one of
their stores located in most medium and large cities.

I've solved the power issue across my several vehicles by
standardizing on the Molex 20 amp, 2 pin nylon connector available
at radio shack.  These connectors will easily accept 10 gauge wire,
more than enough for the 20 amp rating.  The connector is small so
it can be hidden and it can be snapped into a rectangular hole.  I
have some Molex-to-cigarette-lighter adapters made up so that
foreign devices can be operated from any outlet.  For my stuff, I
cut the cord and insert the molex plug in the line to the cigarette
lighter plug.  That lets the gadget run from a cigarette lighter if
I have to use it in someone else's vehicle.  In mine, I simply
separate the Molex plug and plug it in my outlets.

Another option is the Anderson connector.  This is the heavy-duty
connector used on battery packs on forklifts and the like.  Often
seen sticking out the front of wreckers to connect jumper cables
to.  A small version of the connector, rated at 50 amps, is the SB
50 series.  Cost is $4.75 ea from Waytek (800 328-2724)
http://www.waytekwire.com. Waytek is the place to get just about
anything associated with mobile wiring, BTW.

If you don't want to spend that much money, there is another option
that is a little bit better than dime store compressors.  This is
one of the compressors used to pump up air shocks.  You can get one
from a salvage yard from a car like a Lincoln or Caddy that uses air
leveling systems.  JC Whitney also sells 'em.  These are still low
capacity compressors but they're better than the Wal-Mart specials.

John