Jackhammer Forces 2

[bfanger]

I'm working at a company that uses jackhammers to drive our product into the ground. Not much is known about the forces involved except that this size works for this application. We are trying to design a new drive rod and would like to be a little more scientific than we have been in the past. To the best of my understanding jackhammers are rated by weight and impact energy. Neither of which directly leads to a force. The “hammer” is an electric hammer drill with a maximum impact energy of 11Nm. I tried an energy method of-
11 Nm= 97 in lb = ½ k x2
With k being the spring rate of a .625” steel rod 36 inches long – 255,000 lb/in
Then F= x k =7530 lb
This seems quite high for a relatively light piece of equipment

Is there a better way to do this that I’m missing?

 

[hydtools]

If you could determine the mass m of the hammer, you could use energy = 0.5mv^2 to find the velocity v of the hammer at impact.
Then using impulse Ft = mv and assume an impact duration time t=.01 sec you could find force F.
Of course selecting a value for t is the trick. What is the duration of the impact? 0.010 sec? 0.001 sec?
Most customers want to know impact energy. Manufacturers may measure the impact energy. Others may calculate the energy knowing the hammer mass and impact velocity.
Another way would be to measure how deep the rod is driven in a given time. Rod to ground friction force times depth driven would give you the work done. Work done = energy input. The hammer impact energy is per blow. Estimating how many blows delivered times the given rod drive time would give you energy input to drive the rod the measured distance. The result will be an average force, since the first impacts will drive the rod deeper than the last impacts.

Ted

 

[dhengr]

This would be a fun question for a college Physics or Dynamics course quiz, is that what it is? Hydtools... has suggested several approaches to start to get at the root of the problem, but each of them have variables which are tough to put your finger on in the real world. Change the soil make-up or density a bit, or lean on the hammer a little harder, and you have a whole different energy curve. Isn’t the important issue the resistance of the driven rod, against some set of forces, after it has been driven? And, this is still subject to a bunch of variables; actual soil conditions, force orientations, etc. etc., which might be different at every driven rod. Since, in the real world, you only have a selection of a half dozen different hammers, maybe the answer ‘this size works well for that application,’ is a pretty good answer to your question, and at a reasonable cost in time and effort.

 

[Compositepro]

Your method looks close. It gives the maximum impact impact possible. Any movement or energy absorption by the by the object you are working on will result in a lower force. You are seeing why hammers are such effective tools.

 

[bfanger]

I have no real way to know the actual hammer weight without finding a contact at the manufacture of the hammer or taking the one we have apart, which means it probably won't work again. As far as force to drive a rod in the dirt is something I'm looking for but haven't found an answer.

If that 7000 lb is true it does not bode well for the end of rod that necks down to .25".

An idea I came up with is to use some strain gauges on the rod and beat it into the ground. Hopefully I could get a usable strain value and can calculate an approximate force. Only problem is that I don't have any budget to get a data acquisition system right now so I'm still out of luck.

 

[hydtools]

If you have no confidence in taking apart and reassembling the hammer, you need more practice. RotoHammers are not complicated machines. Buy a spare and take one apart.
The strain gage method is not that simple. It took a lot of testing, time, and equipment for the Mounted Breaker Manufacturers Bureau(MBMB)of CIMA to arrive at a strain gage method for measuring breaker impact energy and be accepted by the community of breaker manufacturers.

Ted

 

[dvd]

Seems like the full-load power consumption of the tool divided by hammer impact frequency would give you the maximum work per stroke. (I don't know if your hammer drill is rotating also, or merely hammering.) Stroke length and force can be equated to work. Also depends on the hammer mechanism and force distribution over the full stroke. Of course some power is lost to heat, also.

 

[TheTick]

Maybe put an accelerometer in an existing jackhammer. Use mass of jackhammer, F=ma.

 

[hydtools]

If you use power input to the tool, you must assume an efficiency. Generally 50%, half the power is used to return the hammer to top position and is not available for useful work, the impact energy output. impact energy * frequency/33000 = ft-lb * ipm/33000 = hp Already know the impact energy and the blow rate. bfanger wants to know force delivered by the impact.

Measuring the acceleration of the tool body will give you the reaction to accelerating the impacting hammer. Need to know the mass of the impacting hammer to know the acceleration of the impacting hammer due to the accelerating force to which the tool body reacts.

Ted

 

[GregLocock]

Au contraire, I think the strain gage approach is an excellent way of getting to the root of the problem. In 2 days you'll have a body of work that will put you far ahead of the differential equation boys. THEN you can give the results to them and the equations can be used to help you.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[chicopee]

Something stinks in Denmark!!. A jack hammer to drive a product into the ground or may be, shouldn'd it be a pile driver to drive the product into the ground?

 

[hydtools]

chicopee, it is done regularly, driving rod and anchors with a handheld breaker.

http://foresightproducts.com/pdfs/intro_mr_sr.pdf

Ted

 

[bfanger]

Here I thought I was beeing vage but it is something very similar to this -

http://foresightproducts.com/pdfs/intro_mr_sr.pdf

Funny enough that is a very compeditive market and you don't really want the competion to know what your up to.

Also, the actual value energy value is 11 ft-lb, not that its a teribly huge change. I've also decided to use a spring rate for the product in serries with the rod and that cut the force in about half. It's now in the ball park of my design not failing on paper so I'm just going to act like that is a valid number and work things out in the testing phase. I also am going to push to get funding to do the strain gauge testing later on since I think it would be usful information.

 

[hydtools]

When you do strain gage for impact you will become very good at attaching the gages and anchoring the wire leads so they stay in place during the tests. I believe you can get Labview components for acquiring and recording the strain gage data. It has been a while for me.

Ted

 

[GregLocock]

Yes, alwasy use about 4 times as many strain gauges as you think you need, otherwise it'll spend more time at the lab than being used.

Calibrating them is dead easy, apply a known force to the hammer.

Actually a really neat way of measuring the force would be hydraulically.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[hydtools]

We used about 60lbs of shot to place a weight on the tool in place of depending on an operator pushing down uniformly during a test run.
I'd recommend dead weight for calibration. Simple and always the same.

Ted

 

[GregLocock]

Sorry i meant use a hydraulic load cell instead of strain gauges.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?