How to Calculate Loads on Vehicle Jack Stands

Take the weight of the load, say 6000lbs, and divide by the number of stands, 2 or 4. There you have the static load requirements. If your looking for shock loads, that is dependant on the distance of the drop, 4 inches is different from 6. That again is just math. I had the formula, but it's back at my desk hidden in my machinist handbook.

Laby

I understand the basics ie; jack stands rated at 6 tons per pair can hold up 12,000lbs when used as a pair.

Was more interested in how to calculate shock loads. Reason being as often happens when the floor jack is released the vehicle falls rapidly either due to a cheap floor jack and/or an operator who is in a hurry. This repeated shock loading is what I am concerned about. ie: the math and procedures on how to arrive at the potential shock loads etc. I would guess 4 inches would be a good number to plug in, as most people will jack up the vehicle, then shove the jack stands under the frame or axle, but if they were set a few inches too low, I'm guessing most would not bother to pull the stands back out to readjust them but would go ahead and lower the vehicle onto the stands using the floor jack. I also have heard that jack stands are (or should) be tested not only on the vertical but also at 5 degrees off plumb.

I know of one situation where a jack stand's pawl failed in a static situation causing the ratchet bar to fall down inside the stand, dropping the vehicle's axle on the person's chest fracturing ribs and damaging his shoulder. The stands were rated at 3 tons and the vehicle weighed just under 3 tons. I would assume that some margin is put in when designing the stands beyond their ratings, but that is an assumption on my part. There are of course hundreds of stories of people being killed by falling jacks and stands, but most of those are due to improper use (don't want to get off on that topic however).

Thanks, I'll check back later for more responses.

This is a difficult one. In falling, the truck picks up velocity under the acceleration due to gravity, that means it has kinetic energy. To stop the truck from falling further, a deceleration force must be applied through a distance to use up that kinetic energy. Without a shock absorber, the jack tries to stop the truck instantly. That would cause an extremely high loading. Luckily, the truck has springs and shock absorbers between the bulk of the mass and the axles. The shock load would be modified by these springs and shock absorbers, but I can't take you any further. I hope someone more expert can help with bracketing parameters for the effects of the springs and shock absorbers.

Assume the stands are up off the floor high enough where the front tires do not touch the floor when the jack stand is struck ie: not a factor in the equation. So we are calculating the forces of a 3 ton vehicle falling say 4 (or max 6) inches prior to impacting the ratchet bar at the top of the jack stands. Of course, not the entire 3 tons will strike the jack stands as the rear tires are on the ground. And the rear tires would be supporting a portion of the weight (and the weight is not evenly distributed ie: engine is heavy and up front) and the weight would be traveling in a small or short arc with the rear tires being the pivot point?? Guess it could get complicated, but lets stick initially to how to calculate shock forces and get some rough estimates using the scenario of 3 tons falling 4 or 6 inches coming to a sudden stop with no buffers inbetween ie: metal on metal with the stands sitting on solid concrete.

I don't understand the question. You imply that the stands will be subjected to what can only be characterized as blatant, repeated abuse.

If these are your stands, and your vehicle, and you don't over tighten the jack screw, why would you drop it?

Buy the highest quality jack stand of the size you need and use it properly.

Those pictured look fine to me.

I'm looking for engineering based answers, not comments on how to properly use jack stands. The 1940 version of the Tacoma Narrows Bridge "looked fine" until it fell apart due to wind induced flutter.

Yea,

"engineering based answers"

I have news for you. The bridge was designed by engineers.

If you were designing a jack stand from scratch, I might understand your question.

But, you are stuck with what's out there, so, why do you need, "engineering based answers"?

GA, guest. I think this Roymech website page is worth some study from anyone interested in the subject at hand here; i.e. design for impact. I expect to spend some time studying this stuff myself. I need a good review since the last time I did any design analysis in this area was 15 years ago and even as crude as my work was I'm having trouble remembering anything more than there was some mathematical basis for the idea of a 2x multiplier for the worst impact conditions for simple cases.

Ed Weldon

Suggest you take the nominal worst case load on each Jack for the vehicle concerned - along the lines vehicle weight is 60:40 and Jacks/ similarly unevenly loaded. Then apply a safety factor and design in yield against that. Most engineering failures are in fatigue (usual figure is 10^7 cycle) but that is not likely to be relevant for a jack and in any case can be implicit in the safety factor.

You will have to cast around for a suitable safety factor. Crane and Hoist stuff is very conservative and quite fatigue dominant with as I recall it factors of 8 and 10. Cross country vehicle design is typically 3 or 4 and I'd be inclined to set the factor to about 4 on that basis. You may be able to extract a value out of a hoist or jack standard, or from and SAE document.

Failure can occur in a number of ways. The vehicle coming down on the jacks a little hard for example, but vehicle suspension ( if they are under the axle) or body work deformation (if they are under the body) will limit the maximum value, A factor of 4 will certainly accommodate the former and probably the latter.

But also be aware that there are more subtle forms of failure - for example, if the jack is not vertical there will be bending loads in the neck of the jack, perhaps more importantly extra loads in the legs of the jack. I mention this because one trap for all young designers is "torsional failure" of open sections - i.e. angle and channel profiles. This mode of failure is where the section twists and then fails in bending on a weaker axis. In open or this walled sections there is also the need to be aware of buckling failure.

The engineering math demonstrates that shock loading will result in a significant increase in stress. With that in mind, why not minimize shock loads by lowering the vehicle as slowly as practical and not permitting personnel under the vehicle while it is being lowered? It seems to me that dropping a vehicle at 1.0 g acceleration from any height is irresposible.

I see this as an issue of analysis paralysis. You want to calculate the timely failure of some abused peice of equipment. You don't know exactly how much the equipment is being abused (impossible to figure out) so you're entering an arbitrary maximum. You don't know what the exact load each stand will see (individually) and you don't know the safety factor designed into each stand. Additionally, depending on where the stand impacts the vehicle will have a huge effect on loading (frame vs. body vs. shocks). Additionally you don't know the rate at which the vehicle is dropping (it doesn't just instantaneously begin accelerating towards earth). So I guess what I'm trying to say is, getting an exact answer, which would be difficult and require emperical testing is superfalous (or however you spell that). The REAL issue you should be concentrating on is "How do I get my mechanics to quit dropping the vehicles on the jack stands?" Can you train them better to use the equipment? Or perhaps modify the lifting mechanism so it cannot lower a 2 ton vehicle quickly (whatever quickly is)? I suggest you try a different tact. Yes this is an engineering site. But nobody here would engineer and design an impregnable capsule for crossing the street when we could just follow the rules...

Jason -- You make good points. Especially about controlling lifting mechanisms. I have experienced hydraulic floor jacks with sticky release valves that seem to have no in-between position for on/off.

I don't mean to be flippant here; but try to tell all that to a jury or find the right expert witness (these folks don't come cheap). For someone in business this sort of thing becomes a serious liability issue.

You make a case for simply retiring all the jack stands and buying new. The question then becomes what to buy. I think that is what is at issue here.

To me the technical issue here should not be how to calculate the safety factor for parts of a jackstand that may break. Rather it should be about the properties of the materials used in its construction. Most of the issues of allowable stress levels and safety factors can be set aside if one is assures that there are no materials of construction that are brittle or can become subject to brittle or fatigue failure in service. That means for all practical purposes castings and heat treated steel components are proscribed as are virtually any other metals than low carbon steels with their well known abilities to absorb energy and deflect rather than fracture. (How do you know what they call ductile iron is really that and not some mistake made in parts procurement 8000 miles away?)

Absent the kind of certifications we see in such as crane hooks you should expect your jack stands and other load carrying equipment to be constructed strictly from low carbon steel and steel forgings and be manufactured and performance rated by people that are trustworthy.

Ed Weldon

"constructed strictly from low carbon steel and steel forgings" ......

Hmmmm........ I suppose I should get real here and admit that cast steel, ductile iron or malleable iron is acceptable as long as the manufacturer has a good reputation. Otherwise I think the only source would be specially designed and constructed steel weldments, which is really impractical for most purposes.

Ed Weldon

I really think your best bet with JUST the jack stands then is to institute a fail safe. A pin'ed shaft or something of this nature. I would doubt the material in the stands is very tightly controlled, different vehicles are going to apply wildly different loads under circumstances that are constantly changing (and widely at that). I appreciate your concern, and I agree with your reasoning. I doubt you have a failure analysis curce, but sometimes you have to pick a point on this 'curve' that indicates a failure of less than .2%, or whatever you deem appropriate. So maybe you just replace all jack stands after 5000 hours of use, or whatever. Anything short of applying a load sensor to each and every jack stand will be just a rough estimate, at best. In this particular instance prudence, common sense and a dash of healthy training and awareness are probably your best bet. Good luck either way. No one wants anyone to get hurt. Be proactive and replace the stands if you want to take the safest route. It would be expensive, probably more so then replacing the stands, but you could always do a FPI (florescent penetrent inspection) on the high stress areas... just make sure you scrape away the paint and shine the metal up to a bare surface devoid of any pre-existing cracks inherent to the manufacture process.