2-Post Lift

The only ones I have seen were mounted on a concrete slab and had a manual pin inserted on each post after raising to the required height.

Seemed perfectly safe to me. Not completely idiot proof however. You had to extend and set the lifting pads manually so it was possible to lift the car unbalanced.

I am not very familiar with the pros and cons from a safety perspective to using a 2-post lift versus 4 jack stands, but according to the information at this link:

Inmost case jack stands are required under certain circumstances. B.J. at

According to the tech sheet that accompanied my Eagle 2-post lift, the number one reason for lift failure is the failure to keep the anchor bolts tight.

As long as the lift is correctly installed, the arms are correctly positioned and the car is balanced properly, the lift is as safe as jack stands. Most lift makers offer an optional tripod jack that can be used for added support is some serious "grunting" that might upset the balance is anticipated.

As the other poster said the anchor bolts are the real issue. An off-balance vehicle on the lift turns the lift into a giant lever that will exert many many tons of force on the anchor bolts. And it is easy in that kind of situation to pull anchor bolts right out of concrete.

Keep in mind concrete is excellent for compression, but terrible for expansive forces.

If I was doing one of these in an existing garage I would use a concrete saw and cut out a big footing, then dig down at least 2 feet or more, then at the bottom of the hole put in a big steel plate, and have the bolts for the lift coming through holes in the steel plate, then pour concrete over this. What ends up happening then is all the lifting force on the bolts pulls up on the plate, which then compresses the block of concrete. Also, keep in mind that concrete takes weeks of curing to get full strength.

Ted

If I was doing one of these, I would read the directions for installation. Actually I have done one, and for a 7,000 lb lift, 4 inches of concrete is the minimum requirement. I have no idea why anchor bolts should be an issue unless improperly installed, which is certainly more probable than a manufacturer selling a defectively designed lift.

4 inches in what direction? Depth, width? Concrete also comes in different hardnesses, what were they recommending?

I suspect your directions were for someone pouring a garage floor with the bolts embedded in the concrete. Thus a slab of concrete that's at least 6 foot square - assuming that holds together the leverage would be such that the posts would bend before enough force was transmitted to the slab for it to tilt up.

But if your talking a 1foot by 1foot square hole at 4 inches deep - once the lift was installed, you could probably push it over and the posts would yank that 12x12x4 inch slab up as quick as sin.

People don't understand how easy it is to pull anchor bolts right up out of solid concrete. All you have to do is apply some leverage, which for a thin slab will crack the concrete right through where the bolt is and the second that happens the bolt will come erupting out. That is why you want to have a steel plate down there.

If the vehicle is centered on the lift to where the weight is evenly distributed, then all the force that is transmitted from the posts to the concrete is downward compression which concrete is great at taking. 4 inches is nothing for that.

But if the load isn't evenly centered then this is what can happen:

Ted

installation.

Why not read the installation instructions instead of speculating on the requirements. The instructions for my lift call for 4" thick concrete and assume the floor has been poured prior to installation so anchor bolts are specified. Every 2 post lift I have seen is done thusly. Your suggestion of placing a threaded steel plate under the slab is absurd. And your idea that that a lift will easily fall over is simply unsupported by fact or practice.

Since you didn't supply the requirements I have no choice but to speculate.

As I said already, what hardness are we talking. Is this concrete reinforced or not? What is it on?

How many 2 post lifts have you seen in home garages?

The OP didn't say if this lift was going into a commercial installation or into a garage in a home. The fact that he did say that he was currently using 4 jackstands makes me strongly suspicious that it is in fact a home. The fact that he posted here asking this instead of calling and asking the manufacturer of the NEW lift that he just bought (who have people who's job is to answer these kinds of questions) or is considering buying, also leads me to believe that he has a line on some old lift somewhere he can get cheap - one more indicator of a home user here, not a commercial install.

Threaded steel plate? Where did that come from? I said steel plate, I said nothing about it being threaded.

And you are wrong about it being absurd. Concrete floors in commercial buildings are usually quite different than Bob and Jane's home garage floor. For one thing they use re-rod, or in the newer ones they use a polymer mixed into the concrete that does the same thing. They are also laid on quite a lot of compacted gravel, not just a few rakes of gravel over dirt. (or, just dirt) As a result they aren't sitting on a bunch of soft spots or voids which crack the shit out of them.

You put a couple tons on a 6 inch or 1 foot square area in a commercial building and the concrete just sits there doing nothing. Try that in a home garage and if it's got a good foundation, nothing happens. If there happens to be a void or soft spot under it, you can crack it. That is why so many garage floors in homes have cracks in them - the weight of the car cracks the slab like you would take a board in both hands and snap it over your knee.

In fact, you can crack a home garage floor with a good solid blow from a heavy sledgehammer if you hit it in the right spot.

If that crack happens to intersect the hole you have your anchor hammered down into, that anchor will lose all abaility to resist any kind of real pulling force.

If he is in a home garage and does what a lot of people do and just drills a couple holes and hammers down some anchors, if he puts an unbalanced load on that lift there is a real possibility the anchors could come out. If there are existing cracks in the garage floor that happen to surround where the footer is, the entire section of the slab in the cracked area could start leveraging up.

Then you have no explanation for the jpg that I posted which clearly shows a destroyed concrete footer by a post using bolts a lot bigger than any that a lift would use. Wouldn't you like to know how that happened? I'll tell you, it was during the construction of a building, by people who quite obviously had no better concept of a lever than you have shown. Several people missed being killed by inches on that job. Sadly, almost the same thing happened a few years back during the expansion of the PDX airport parking garage and 2 people did lose their lives as a result.

Your assertion that a 2 post lift won't easily fall over is what is unsupported by fact. It can easily fall over and the only thing keeping it from doing so are the anchors. If the anchors are set by someone who understands what they are setting them into, they can resist many tons of pulling force and you will not even notice even if the load on the lift is grossly unbalanced. If they are set by someone who doesen't understand anything about what he is putting them into, and is just blindly following some instructions, then you better hope to hell that they never puts an unbalanced load on that lift.

Ted

Here ya go asshole. Other lifts are similar.

Super, thanks very much for all those responses. The lift is to be placed in an Auto Club and we have not purchased a particular brand yet. Of course cost is an overriding factor.

What I am hearing is that "IF" the 2-post lift is installed IAW mfg instructions that it is safer than a car on 4 jackstands. Did I hear that?

Mike

Both are as safe as the operator makes them. The lift is waay more convienient, but costs a lot more. I bought a good used lift for $1400 and it is worth every penny. Plus I get to see friends I haven't seen in years. Make sure you have room to install the lift esp. hight and remember the side post might require more vertical space to stand it up.

Let's do some math, shall we? Assume basic concrete has a strength of around 4,000PSI (usually higher, though). A one foot square base plate with 1,000# of hoist and say 4,500# of car (since many lifts are rated to 9,000#) on it is going to exert a compressive force of around 458# per square inch. That's well below the 4,000lbs/square inch, wouldn't you say? Even with a safety factor of 5 you're still only half the load allowable on even cheap ass 4,000PSI concrete. Oh, but you'll say that the tensile strength of concrete is only about one tenth of compressive strength, and that's right! Wow! However, even if you managed to get the car off center enough to seriously load the base without the car falling off the hoist about the most tensile load you can expect is going to be a couple hundred PSI, and I think you'd really have to work at that by welding the car to the hoist arms with a hell of a cantilever to even get that.

You can crack a shop floor with less. Been there, done that. So what? The hoist isn't a sledge hammer, and the sledge hammer isn't a hoist. For one thing the sledge hammer is imparting force into a much smaller contact area than any hoist possibly could. Would you rather get hit by a 5oz knife tip or a 5oz baseball?

Duh, so don't whack the floor with a sledge hammer, silly!

So, what does that picture of a structural building collapse have to do with two post hoists? Near as I can tell, it looks like the collapse happened while they were still putting up the roof trusses, before the interior slab was poured and before the structure was more than a shell. That blue H-column doesn't look like any hoist I've ever seen, and in fact it doesn't appear to be any kind of a hoist since it's missing the lower mechanism, arms, hydraulics, etc. In fact, I would guess that whatever caused that building to collapse (why don't you post complete details about that, anyway?) ended putting more force on that column than it was designed for. Sure, I'll throw 8 tons of horizontal force on a column and footer designed to only handle 4 tons of vertical force, then be surprised when it fails. LOL!

Maybe you should take a beginning class in statics and dynamics? I found that both Strengths of Materials and Engineering Materials to be very enlightening, it would sure fill in some obvious holes in your knowledge.

Oh, and before you get your panties in a wad, remember, this is the internet and nobody cares what you think, not even me. :)

JazzMan

Jazz .... cool

So you are saying that the 8 bolts into a 6" slab is sufficient enough and we shouldn't be worried about ( or fuzz about ) the lift tipping over?

Mike

I would say that the manufacturer's instructions and mounting specifications would be the way to go. I don't know how many bolts are used in this particular installation and for the purposes of the discussion it really doesn't matter. The manufacturer has the experienced engineering staff on hand to actually determine what is appropriate, and I'm sure they take into consideration liability issues when calculating safety margins and such. I did some more thinking about the situation today at work and figured that it would take almost a deliberate act of misuse and severe screwups on installation to create problems.

Again, I'd go with the manufacturer's installation instructions.

JazzMan

As you can see by the posts here the installation of a two post lift, or in fact any lifting device that is not portable is going to be critical to the safety of the installation.

There are a number of lifts priced under $2000 advertised in the various free Auto for Sale type magazines.

One is

which is in Indianapolis, IN.

They should be able to give you installation requirements.

There may also be local and state building codes that apply to this type of equipment.

regards

I already said that is not an issue. I'm glad you had some fun calculating that out for yourself.

As soon as it's off center the compressive force is no longer evenly spread over the base. So that 458 psi now changes to much lower on one side of the base and much higher on the other. And although that may not be enough to do much, if there happens to be a void under where the lift base is, regardless of whether it's rocking around or not, that means the weight of the vehicle is on that - not in psi, the entire weight. Your dependent on how well the concrete was poured, they didn't over water it, there isn't a big air bubble in that section, it was properly cured, there's not some other existing crack that ruins the slab strength. I've seen some really horrible stuff poured for garage floors in new home construction to simply not trust it.

And, all that's just assuming you might crack it. You could still pull an anchor out even if it wasn't cracked if the anchor wasn't set right or the wrong kind was used.

In any case, this is speculation that was finally answered as the OP provided in a subsequent post, it's not going into a home, it's going into a commercial structure.

It was there simply to illustrate that you CAN pull anchors out of concrete, really big ones. You and Chas both implied it wasn't possible to do so.

I've got more pics of it. The interior slab was poured, they had the roof trusses up. But, you can clearly see why it collapsed, there's not a single cross brace in the entire thing.

I would assume that even a rank amateur architect wouldn't make that kind of a mistake so I would guess this was an erection error by the contractor. This happened in Sherman TX and I supposed what else do you expect there but a bunch of cost cutting. I'm sure the contractor had figured he would save money by having them put in the cross braces after the roof was up.

These came from a business that a friend of mine worked at, and shortly after they were distributed that company put a lid on all discussion on the project - quite obviously they decided to sue the contractor, the all too common result with that kind of c*ck up. They had the building up a month or two later so that's why I'm prettty certain it was an erection problem, if the inspectors had found a design flaw it would have had to go back through design and would have taken a lot longer. As you might guess my friend never heard anything from anyone else at that company as to what the problem had been, I think they all tried to pretend it never happened.

Then, why do we both bother posting? :-)

Ted