Calculation of stress caused by an impact 2

[thedillweed]

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?

 

[chichuck]

'weed,

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

 

[GregLocock]

I agree that a dynamic analysis is appropriate, but I see no reason why you can assume that 1g should be regarded as an upper bound on the acceleration, unless you have previously analysed a similar system.

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

 

[PXC]

I would have thought that every crane would have rubber (or hydraulic) buffers at the ends. If it doesn't, it needs to. You need to find out the performance data for the buffers.

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.

 

[BreadTruck]

Design of Welded Structures by Omer Blodgett is a good reference and should put you in the ball park.

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

 

[Denial]

Chichuck, in his post of 3 Sep 03, states: "If my memory serves me, the upper limit on the dynamic force due to a "sudden" impact is 2.0 X force."

This rule applies to "suddenly applied" loads, but not to "impact" loads.

 

[thedillweed]

Thank you all for your advice and suggestions. They have been very helpful. I think I am going to try to find that book by Blodgett and do some in-depth reading on the subject. The crane does have bumpers, by the way. I forgot to mention that in my original post. They are solid polypropylene, about 8 inches long, so that will give me some cushion during impact. BreadTruck, you wouldn't happen to have an equation for solid bumpers instead of spring bumpers would you, or would it be the same? Thanks again!

 

[Lutfi]

Better yet get the CMAA 70 and 74 code books. They have very detailed method and formulas on how to compute the crane stops. Dr. Blodgett’s approach is identical to theirs.

Also AISC published industrial building design guide/recommendations that tackle the cranes and crane stops.

Good luck.

 

[unclesyd]

Omer Blodgett's book is available from:

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.

 

[BreadTruck]

thedillweed,

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

 

[unclesyd]

For so good spring calculators:

http://www.efunda.com/DesignStandards/springs/spring_calculators.cfm

These are a very handy set of calculators that will save tour fingers.