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Reciprocating Saw Impact Force

Reciprocating Saw Impact Force

Reciprocating Saw Impact Force

(OP)
Hi,

I am try to figure out how to find the impact force that a Recip saw (Sawzall) can exhibit.

To better understand:

If you took the blade out of the saw and just used the reciprocating rod (.5" Dia) was held 1 inch off a block and then the trigger pulled so the shaft smashes into the block.

I had asked a friend of mine in materials and he thought you could use:

Power=Force*Velocity=Current*Voltage

And measure the peak current of the saw. Does this make sense?

Thanks for your help.

dreinert

RE: Reciprocating Saw Impact Force

Sounds like a junior or senior level homework problem to me. Is it? Or is this really an on the job issue?

jt

RE: Reciprocating Saw Impact Force

Must be an electrician, using a saw to drive nails.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Reciprocating Saw Impact Force

lol, saw to drive nails. That cracked me up.
P.S. That doesn't make sense to me. You would need to know a bit more then that to figure out the impact force. You could reference mechanical engineering handbook for example.
Alternatively, a quick solution is to strap a force transducer to something and 'wack' it with your saw. However, this method is not the best, just the quickest.

peace

Fe

RE: Reciprocating Saw Impact Force

Why do you need to know the exact force?

pipe

Fe

RE: Reciprocating Saw Impact Force

The equation wouldn't work anyway.  The peak current occurs at maximum acceleration, not maximum velocity.  Besides, your force*velocity are both unknowns.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Reciprocating Saw Impact Force

If you know the rpm of the crank and the eccentricity of the driver you can calculate the velocity.  Weigh the parts.
Then you have what you need to calculate energy anywhere in the stroke.
Or you know the strokes per minute and stroke length.  You can calculate velocity.

Ted

RE: Reciprocating Saw Impact Force

Quote (Mike Halloran):

Must be an electrician, using a saw to drive nails.

C'mon Mike, even a mechie should know that's what a hammer drill is for. Why carry two tools when one does both jobs. lol
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Reciprocating Saw Impact Force

Point it at a scale and fire.

RE: Reciprocating Saw Impact Force

(OP)
Hi guys,

Thanks for the input.

The issue with the saw is that after it experiences several hard impacts gear teeth start breaking off.

I was thinking like TED and thought that with the motor specs and gear ratio we could calculate the velocity of the recip rod.

I also thought that upon the impact of the saw the amps would peak because the motor would stop "instantly" and still be appling power, operating higher than rated.


The problem with the transducer is that a force transducer setup is going to cost me $600+, unless anyone know of a cheaper place.

Thanks for the help.

dreinert

RE: Reciprocating Saw Impact Force

Now you know the saw will not last as a nail gun.

There is no give in the drive to allow the shaft to impact while the drive follows through.  Everything in the drive comes to an abrupt stop at impact.  Eventually something breaks.

One method used to estimate impact is to drop a steel ball onto the reciprocating shaft and measure how high the ball is thrown.  Using the ball weight and height reached you can calculate the energy trasferred to the ball.  Be careful.

Ted

RE: Reciprocating Saw Impact Force

dreinert,

   Impact is not just a straight force calculation.  You are developing kinetic energy in something, and then dissipating it into whatever it is you are hitting.  The force is affected by the response of whatever it is you are hitting, and probably by the mass of whoever is holding your reciprocating saw.  

   The quick and dirty answer is that your impact is the abuse that voids the warranty of your saw.  The long, complex answer requires a detailed study of what is holding your saw, and what you are hammering.  It is probably a waste of time.  

               JHG

RE: Reciprocating Saw Impact Force


dreinert,

It seems obvious that the saw wasn't designed to put up with these "hard impacts", so what's causing them and can't they be eliminated?

If not, would it be possible to fit some protection for the gears in the form of a shear pin that will let go before the gears break or an adjustable slipping clutch that you find on the feedscrews of some lathes, you could adjust this until it transmitted enough power to cope with the worst cutting conditions but would let go on "impact".
 

RE: Reciprocating Saw Impact Force

The accelerometer may be cheap.

The power supply, amplifier, and FFT-capable recorder aren't.

RE: Reciprocating Saw Impact Force

dreinert (Mechanical)
Since a Milwaukee Sawzall costs a little over one hundred dollars, Wouldn't it be cheaper just to give the operator another saw?
 Then if the guy keeps breaking the saws, replace the operator.
B.E.

RE: Reciprocating Saw Impact Force

Make the gears stronger than the blade....after a couple dozen blade changes, the operator might decide to change his technique.

RE: Reciprocating Saw Impact Force

The saw is handheld, which is all that mitigates the impact.
Dreinert's problem is that indestructibility is a big part of the Sawzall brand.

If he's working on warranty issues for the real Sawzall, then he has to find and reverse the last cost-saving initiative on the gears.

If he's working on a faux sawzall, then he hasn't quite got it right yet.




 

Mike Halloran
Pembroke Pines, FL, USA

RE: Reciprocating Saw Impact Force

What about decreasing the spring rate and increasing the travel of the rest piece that is supposed to be flat against the cutting surface.  This is what absorbs the impact force when the saw binds and is pulled towards the work piece.  If the saw is forced away from the work piece the impact is likely less as it is absorbed by the operator.

One test would be to reduce the cross-section of the blades so that the blades start to fail when the saw binds/grabs.  Then compare blade tension and/or Charpy tests to get an idea how much energy is involved.

RE: Reciprocating Saw Impact Force

Dreinert, if you study cam motion, velocity and acceleration curves, particularly acceleration curves, you can estimate the forces incurred during the reciprocal motion of a blade.
Obviously the impact is not as significant as lets say a bullet hitting a surface, the damage would still be significant due to the penetration of a thin blade.  

RE: Reciprocating Saw Impact Force

Dynamic loads are tough to measure.  I doubt there's any relationship between the motor current and the peak load at impact.  I'd guess that most of the load is from decelerating the moving mass. Accels are easy to mount and small - but if you want "force" then you would have to estimate how much mass is being accelerated. That might not be so easy if gears are accelerating and their teeth are bending. Load cells have mass themselves and will have to be inertially compensated to add (or subtract) any outputs from accelerating the load cell (or parts of it).  I'd consider a piezoelectric load transducer (www.kistler.com for example) if you go with the load cell idea.

What do you do after you measure the load?  If you change your device and if any of its mass or accel or springiness changes then the load you measured with this setup won't be the same load you measured with the new setup.  

How stiff do you make the impacting surface?  For worst load cases it would need to be stiff (thick and well supported).  But that would be a bit unrealistic for most cases in the field.

Have you considered strain gaging the rod and calibrating the strain with a known static load?  Or if the the rod is a simple section you would'nt need to calibrate it.  You'll still get some inertia effects but if it's a simple rod then the inertia effects can be easy to estimate within 5-10% I'd think.

RE: Reciprocating Saw Impact Force

You said you wanted impact force at the rod tip from impacting a rigid "target". You could softly suspend a metal block so that it moves freely in the horizontal direction, and mount an accelerometer on the back side. Force = mass x acceleration, and you can weigh the mass of the steel block plus accelerometer. An O-scope or spectrum analyzer could measure peak acceleration and then calculate force. You would not be measuring worst-case force by the method you suggested, because you are appling a transient where the entire drive mechanism is accelerating when you pull the trigger. Maximum speed (impact velocity) is probably not reached when impact occurs. Unless that's what you really care about. There are other measurements and calculations that could be used to get to the root cause of the drive gear failures. I own a Sawzall, and I use a lot of dynamic measurement instruments for my independent consulting services for machine diagnostics.

Walt
w_f_strong@msn.com

RE: Reciprocating Saw Impact Force

I like the strain gauge idea. Seems to be the most realistic. You could even strap a strain gauge to the rod and use it for whatever you are doing with it.
 

Fe

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