Reciprocating Saw Impact Force 1

[dreinert]

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

 

[chicopee]

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.

 

[BobM3]

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 (

http://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.

 

[Strong]

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

 

[IRstuff]

Sure, there are lots of internet discussions about ballistic pendulums.

TTFN

FAQ731-376

 

[FeX32]

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