Jackhammer Forces
Jackhammer Forces
(OP)
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?
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?





RE: Jackhammer Forces
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
RE: Jackhammer Forces
RE: Jackhammer Forces
RE: Jackhammer Forces
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.
RE: Jackhammer Forces
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
RE: Jackhammer Forces
RE: Jackhammer Forces
RE: Jackhammer Forces
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
RE: Jackhammer Forces
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Jackhammer Forces
RE: Jackhammer Forces
http://foresightproducts.com/pdfs/intro_mr_sr.pdf
Ted
RE: Jackhammer Forces
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.
RE: Jackhammer Forces
Ted
RE: Jackhammer Forces
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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Jackhammer Forces
I'd recommend dead weight for calibration. Simple and always the same.
Ted
RE: Jackhammer Forces
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?