When I was a young man I was taught that every action has a reaction. 
If I use an electric drill to bore a hole -  and the drill bit becomes
'stuck' the electric drill tries to twist my arm.   If I use an 'air
powered' mechanics' gun I am able to undo ever the tightest nuts/bolts
e.g. lorry wheel nuts,  with hardly any effort  -  apart from a
'machine gun' noise.   My query   -  WHERE HAS THE REACTION GONE THAT
NEWTON SAID THERE WOULD BE?

                Regards,

                    Brian Allanson.

What seemed to be lost in the comment/conversation in this question
are three things, keeping in mind that these tools are called IMPACT
wrenches (at least in the USA):

1) The wrench works by taking advantage of a large angular momentum
(the impact).
2) Angular momentum is conserved (Newton's second law).
3) Torque is balanced (Newton's third law of motion... when you try to
turn the lorry nut, the nut pushes back!)

The energy that drives the wrench is the energy released by air that
is decompressing through a small turbine like device that turns the
shaft of the impact wrench. Now, despite the few moving parts in an
impact wrench, it is an extremely complicated device to examine
analytically. So, right here, lets take the spin-up of the wrench out
of the picture (this is the small torque that you feel when you
activate the wrench in mid air)... let's assume that the wrench is
simply operating in a steady state.

On this spinning shaft are tiny little hammers (see the following
link):

http://groups.google.com/groups?q=%22impact+wrench%22+hammers+turbine&hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&selm=387dd777.1689053%40news.iconnect.net&rnum=1

As stated in the above link, these tiny little hammers are spun around
this shaft and are supported on a yoke. Due to centripetal force, the
hammers are pulled away from the center axis. This position farthest
away from the center axis gives them a large angular momentum
(dependent on the spin up of the tool). When the wrench encounters a
nut (which trys to slow down the shaft), the hammers are pulled
inward, smack a yoke and give the nut a very large impulse of angular
momentum.

This impulse takes advantage of the concept of stick/slip friction and
can "break loose" a very tight nut. The "breaking loose" occurs when
the impulse is able to overcome the sticking friction of the nut. Once
the nut is moving, the force needed to keep it moving is relatively
small.

So... the answer to the original question comes down to this... the
impact wrench operates in two different modes:

1) Hammering
2) Spinning

When the wrench is hammering, the torque on the hammer is applied to
the nut. Now, despite the large impulse that the internal hammers are
imparting on the nut (and the consequent REACTION on the wrench
itself), there is not enough of a torque developed (during this
impulse) to move the relatively large inertial mass of the wrench (not
much anyway).

When the wrench is simply spinning the nut, their is a torque
developed that is dependent on how much torque is needed to keep
removing the nut. But, as I wrote already, there is not much torque
needed once the nut is spinning freely.

Another monkey wrench for you (pun definitely intended), the shaft is
not directly connected to the wrench... what I mean is that the
impulse that the hammers provide and the torque that is developed on
the nut is actually provided by the compressed air. What this means is
that the balancing force needed to keep the wrench from flying out of
your hands is due only to frictional forces on the shaft (and any
misalignment with the nut). The spinning turbine imparts pressure
waves into the air hose feeding the impact wrench (you can notice
cheaply made air hoses jumping around a little). These pressure waves
combine with the hammers smacking the yoke of the device to produce
the machine gun sound you wrote of.

I hope this answers your question Brian, please ask for clarification
on anything that you believe i did not cover in depth.

krobert-ga

I have to disagree with the GA's answer on this one.  He/she states:

>In the process of compressing air, much of the "reaction" created
>energy is expended as heat rather than as mechanical energy.  Check
>the temperature of a bicycle tire after you pump it up.

It is conceptually incorrect to introduce the notion of energy into 
Newton's 3rd Law.  Newton's 3rd Law states that "every action will
have an
equal but opposite reaction".   The action Newton is talking about is
*force*,
not energy.   Force is a vector quantity while energy is not.

The simple answer to the question is that the air tool in question has
a longer
lever arm, and thus the user exerts less force where he is gripping
the tool to counter
the torque imparted on the nut or bolt.

The source of the energy, be it from a battery or electrical
generating plant 500 miles
away or from a compressed air reservoir, is not important at all.

The answer given is conceptually incorrect.

andy22

While both the answer and comment are conceptually right depending on
interpretation, I am asking that my answer be removed.  Either way,
Newton's Third Law applies, the action, reaction process didn't
vanish, it still exists.

Force cannot be applied without the expenditure of energy.  And air
pressure is definitely 'force.'  If the bulk of 'force' or energy is
utilized or expended in a remote location, such as moving a cylinder,
it is no longer available as reactive force or energy in the tool.

Hi digsalot-ga,

Wait. Before asking the removal of your answer you could make it sure
that no valves and compression are used to dimish that reaction.

regards,
g

meant "diminis" instead of "dimish".

Yeh. Andy22's explanation seems too ...

The thing about pneumatic wrenches that makes it easy to loosen tight
nuts is the hammering action.  If you try to loosen a tight nut with
an electric drill, it is likely to twist out of your hands.  Each time
the wrench "hammers", a very large torque is applied to the nut for a
very short time.  A torque of the same magnitude but opposite
direction is applied to the wrench for the same amount of time (by
Newton's law, as you pointed out).  Though this torque is, as I said,
very large, it acts for such a short time that it can't give much of
an angular velocity to the wrench.  In physics, the amount of momentum
you give something is called the impulse.  Impulse is equal to force
multiplied by time.  If you think of a 'rotational impulse' which is
the same thing, but for rotational rather than linear forces, then
rotational impulse is torque multiplied by time.  The rotational
impulse of each 'hammer' of the wrench is fairly small--enough angular
momentum is transferred to budge the nut, but the wrench is much
heavier, and so the amount of angular momentum transferred to it only
moves it a little.  Basically, a large amount of torque is needed to
turn the nut, but the wrench supplies that torque in short bursts. 
The rotational inertia of the wrench allows you, the operator, to
apply a much smaller torque spread out over a longer time.

"pneumatic wrenches"

http://www.atlascopco.com/tools/products/website.nsf/0/2cd20ab5cce54542c1256c5c002fc555/$FILE/Tightening%20technique.pdf

pages 17- 22
pages 18-19:
"  The advantage of impact wrenches is that they have a very high
capacity
relative to the weight and size of the tool. As the reaction torque is
not
greater than that needed to accelerate the hammer the reaction force
transferred back to the operator is very small..."

http://www.spairtool.co.uk/pneumatic_tools/wrenches/
:
"Applications:
Excellent choice for heavy automotive and truck work such as tyre
changing, shock and spring work, farm, body shop, and front end work.
Ideal anywhere extra powerful tools are needed.
 Features: 
Twin dog clutch provides more power and smooth balanced blows  
"

I have to say that I think the answer given is patently incorrect.  If
the given answer were correct then a battery operated impact wrench
would not work because "both the action and the reaction are contained
within the same tool."  The impact wrench operates in the same manner
as a hammer is able to insert a nail into a board.  It is the brief
impulses that cause the nut (or nail) to move without accelerating the
impact wrench (hammer) out of one's hand.  The two devices work
against the inertia of the tool.

"The power with which this takes place is proportional to the diameter
of the piston - the greater the diameter the more power."

This statement is incorrect.  The amount of energy (from which follows
power) in the system is constant.  It is the force which is
proportional to piston diameter (for a given pressure).  If you demand
greater force through a larger diameter piston, then you will have
reduced piston velocity as the energy must balance.

Bradley

I have been digging through other resources.  It seems the material I
was using was dealing with theory only rather than the actual
mechanics of the way the tool works.

Therefore, my answer is wrong.  A new request for removal has been
made.

To all who jumped in on this, my thanks.

Cheers
digs

Hi eek-ga,
By what I have learned here from racecar-ga then my web-findings about
pneumatic wrenches is that the
 "accelerating the impact wrench (hammer) out of one's hand" 
is prevented by proper limit settings, clutch/es (for decoupling)
and the small accelerating torque --among others probably.

googel-ga

Hi eek-ga, 
What I have learned here from racecar-ga then my web-findings about
pneumatic wrenches is that 
 "accelerating the impact wrench (hammer) out of one's hand"  
is prevented by proper limit settings, clutch/es (for decoupling) 
and the small hammer-accelerating torque and force --among others probably. 
 
googel-ga

To any researcher,

Please answer this question and feel free to use any of my comments
here.

http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=/netahtml/search-adv.htm&r=9&p=1&f=G&l=50&d=ptxt&S1=((wrench+AND+reaction)+AND+(force+OR+torque)).ABST.&OS=abst/(wrench+and+reaction+and+(force+or+torque))&RS=ABST/((wrench+AND+reaction)+AND+(force+OR+torque))

United States Patent  6,009,775  
Inventors:  Thompson; Owen R. (Louisville, KY); Weber; Edward J.
(Girard, PA); Rounds; Jerry L. (Erie, PA)
Assignee:  Titan Tool Company (Fairview, PA)  


2. Background of Related Art 

A wrench that transfers energy stored in a flywheel to a bolt or nut
which is to be loosened is conventionally known in the automobile
lugnut removal field. U.S. Pat. No. 5,158,354 to Simonin discloses a
conventional wrench with a drive motor and flywheel that are rigidly
connected in a housing to drive an output ferrule when a spring clutch
is engaged. In operation, a user provides power to the drive motor
which causes a flywheel to rotate. Once the flywheel achieves a
predetermined speed, the user presses the output ferrule onto a lugnut
which causes a single tooth clutch plate connected to the ferrule to
collide with a mating single tooth clutch plate connected to the
flywheel. The rotational energy from the flywheel is then transferred
to the output ferrule to provide a removal force to a lugnut engaged
by the ferrule. The conventionally known wrench is designed for the
specific purpose of quickly removing a flat tire. Accordingly, the
conventional wrench is designed to be economically made with little
concern for accuracy or endurance.

Because the motor of the conventionally known flywheel wrench is
rigidly connected to the housing, a torque reaction will be
transmitted directly to the user of the device. Torque reaction is a
detrimental reverse torque which results from the elastic collision of
the clutch mechanism when the rotational energy transmitted from the
flywheel to the output ferrule is converted to a torque for removing a
fastener. Transmission of torque reaction to an operator can lead to
many undesirable health problems including nerve damage, muscle strain
and bruising. Torque reaction is especially large when the rotational
energy stored in the flywheel is not sufficient to remove the fastener
to which the output ferrule is connected. Torque reaction is also
compounded when any of the mechanisms that are rotated are not
concentric. The nature and object of conventionally known flywheel
wrenches has never demanded a strict limit to the amount of torque
reaction that is acceptable because conventionally known flywheel
wrenches are generally used in lightweight limited use applications,
such as removing a lugnut from an automobile wheel. Accordingly, the
detrimental effects of torque reaction being transmitted to an
operator are negligible in conventionally known flywheel wrenches and
do not outweigh the benefits of making the device economical and
compact.

In heavier, industrial applications, it is conventionally known to use
an impact wrench to remove fasteners. The impact wrench also suffers
from the problem of transmission of torque reaction to the operator.
In addition, the user of an impact wrench has little control over the
amount of torque that is output by the tool. Torque output from air
operated power equipment, such as an impact wrench, varies greatly
depending on the air pressure, amount of moisture in the air and the
condition of the motor itself. Furthermore, impact wrenches require a
relatively large amount of input power to achieve a given output
torque.

SUMMARY OF THE INVENTION 

The present invention has been made in view of the above problems. An
object of the invention is to provide an economical and efficient
wrench that transmits little torque reaction from the output drive to
the wrench housing. Another object of the invention is to provide a
wrench that can be easily and accurately controlled to provide a
specific torque output. A further object of the invention is to
provide a wrench that can be used while suspended by a cable without
requiring the physical control of an operator during use. Yet another
object of the invention is to provide a wrench that requires a small
power input to achieve a large torque output.

According to a first aspect of the invention, there is provided a
power driven wrench in which a drive motor is located inside a
housing. An inertial mass for example, a flywheel is connected to the
drive motor such that it can be rotationally driven. An output drive
mechanism is located at an output end of the inertial mass. The
inertial mass and the drive motor are connected to the housing such
that they can rotate with respect to the housing to substantially
prevent torque reaction from being transmitted to the housing.

Hello krobert-ga,

Yours is a real nice explanation given to those cracking their nuts on this
question of Brian.

Thank you.

from Question ID: 223701 :
Subject: to resrchr krobert-ga for answering brian0575-ga's "Newton's Law"--- 

Asked by: googel-ga 

Expires: 30 Jul 2003 15:43 PDT 
Question ID: 223701 
 
Thank you, krobert-ga, for answering brian0575-ga's "Newton's Law"
question and thus satisfying my curiosity about the working principle
of impact wrenches.

   Clarification of Question by googel-ga on 30 Jun 2003 15:58 PDT 

     Thanks to brian0575-ga for the question: 
 
     Subject: Newton's Law