Calculation of stress caused by an impact
Calculation of stress caused by an impact
(OP)
I am modifying the endstops for a large overhead crane in our shop. The crane weighs about 80,000 lb and I assume the maximum speed when it hits the stops to be 120 feet per minute(this is a constant velocity, i.e. zero acceleration). Of course, the operator should never actually be going this fast when he hits the stops, but you never know! The endstops are constructed as upright steel built-up beams, the cross section of which is basically like an i-beam (moment of inertia for this section is 1035 in^4). The stop is 31 inches tall, with the point of impact occuring 23 inches from the base. I need to calculate the stress exerted on the endstop during such a collision to make sure that my modifications will not result in failure of the stop. What is the best way to calculate this?






RE: Calculation of stress caused by an impact
I'd suggest looking into a structural dynamics book. Two I use sometimes are by Mario Paz, and one by J. M. Biggs. If my memory serves me, the upper limit on the dynamic force due to a "sudden" impact is 2.0 X force. In this case, I think the force is the inertial force at each end of the bridge. The absolute upper limit for this inertial force I think is m*g, or the weight at each end. So, an absolute upper limit would be Summ(wheel loads) * 2 for the lateral force on each stop. The lateral force is something less than m*g, so the above is very conservative.
chichuck
RE: Calculation of stress caused by an impact
Perhaps the original poster could work out the stiffness of the post as seen by the impact and then model it as a SDOF system?
Cheers
Greg Locock
RE: Calculation of stress caused by an impact
The way to solve this problem is to equate the kinetic energy of the crane with the work done in compressing the buffer, which is the area under the force/displacement curve of the buffer. This then gives the force on the endstop. The larger the stopping distance, the smaller the buffer. If the crane is travelling too fast, the buffer may not be able to stop it before it runs out of travel, and you may have to revise your crane design speed. ie you may be too optimistic to expect to stop the crane at full speed.
If you do not include a buffer and rely on steel to steel contact, the stopping distance is due to elastic deformation of the steel which is very small, then plastic deformation, which has the ability to absorb a lot of energy.
This energy approach is appropriate to all impact problems. It is covered in Blodgett. (Lincoln arc welding .... I recall). It is not well written but it is all there and it is worth struggling with.
RE: Calculation of stress caused by an impact
For spring-type bumper blocks, the longitudinal crane stop force may be calculated from the following formula:
F=WV^2/gc
W = Total weight of crane exclusive of lifted load
V = specified crane velocity at moment of impact, fps (required by AISE Technical Report No. 6 to be 50% of full load rated speed).
c = stroke of spring at point where the crane stopping energy is fully absorbed, ft.
F = Total longitudinal inertia force acting at the elevation of the center of mass of the bridge and the trolley. The force on each runway stop is the maximum bumper reaction from the inertia force acting at such locations.
g = acceleration of gravity, 32.2 fps^2
Also note that in the absence of specific data, it is common practice that the designer assume the bumper force to be the greater of:
1. Twice the tractive force, or
2. Ten percent of the entire crane weight.
Hope this helps!
BreadTruck
RE: Calculation of stress caused by an impact
This rule applies to "suddenly applied" loads, but not to "impact" loads.
RE: Calculation of stress caused by an impact
RE: Calculation of stress caused by an impact
Also AISC published industrial building design guide/recommendations that tackle the cranes and crane stops.
Good luck.
RE: Calculation of stress caused by an impact
https://ssl.lincolnelectric.com/foundation
The cost is $12.50. The also have some other very good reference books at equally good prices.
Check Section 3.1 for you impact loading.
RE: Calculation of stress caused by an impact
The above equation is not directly applicable. Blodgett's reference will help. It is a little cloudy at times [at least it was for me] but it is worth the absorption. I would look at relating the stroke of the spring to the deflection of the bumper and the trasfer of kinetic energy to potential.
Manufacturer's literature or experience is the only other option for this type of installation.
Sorry for the delay, been busy.
BreadTruck
RE: Calculation of stress caused by an impact
http://www.efunda.com/DesignStandards/springs/spring_calculators.cfm
These are a very handy set of calculators that will save tour fingers.