2 post lifts and torque values

[sbhunter]

I have seen a lot of questions and comments about these 2 things lately and have not seen a couple important qualifiers. I have no formal training in either of these subjects, just an old farmer, so feel free to correct me or add input.

2 post lifts by their nature apply a lot of prying force to the surface they are bolted to. My lift required 8” of reinforced concrete minimum. It is a 9000lb 10’ lift. If the lift you are looking at is a free standing version with no plate or bar connecting the 2 posts at the top, make sure you read the literature about concrete thickness, tensile strength and minimum reinforcement in the concrete before you purchase it. The average residential garage floor is not not designed for this installation.

I was raised to always grease bolts or studs when assembling something. As my grandpa taught me, “If you think you will ever take it apart, grease it!” Any type of thread lubricant used during assembly significantly alters the torque value needed to tighten the bolt or nut. I believe torque values are established with dry threads. The style of bolt or nut also affects torque value. For example, if you change a standard nut to a flange head nut, the torque required to keep it tight is much less. This of course depends on whether the torque rating is aimed at the stretch of the bolt or the friction required to keep it from backing off.

[edwardb]

You asked for input, so here's a little.

I've installed two 2-post lifts. A Danmar and a Bendpak. Both on residential garage floors. Both came with very specific instructions for the cement floor where they were installed. I'm surprised about your 8-inch requirement. I agree most residential garages won't have that. But neither of mine specified anything like that. My 10,000 lb rated Bendpak said "4” Min. Thickness / 3,000 PSI" and the usual comments about being level, not cracked, not on expansion joints, etc. I did some checking, and found that unless the house was old 3,000 PSI isn't an issue. Most, especially in colder climates, are more than that. Mine was built in 2001. When I drilled for the inserts, I was past 6 inches before breaking through. Everyone needs to do their own due diligence because obviously it's very critical. But the general statement that the average residential garage floor won't handle a 2-post isn't always the case. Another comment (and I'm not an engineer either, so take it FWIW) is that the forces other than just prying forces. My Bendpak came with very specific instructions about where to position cars on the lift, and huge warnings not to lift on only one pair of arms. Obviously balance is important, which would mitigate prying forces to some degree. I'll never have anywhere near 10,000 lbs on mine. But I'm confident it would fine it I did.

Regarding torque values, agreed adding lube changes things. Best advice IMO is to follow instructions. When I did ARP headbolts on my SBF rebuilds, they came with very specific instructions for the exact kind of lube to use (ARP of course) and the exact sequence to tighten. In general though, unless stated otherwise, torque values are typically given dry. I'd be a little careful about lubing a bolt and then applying a dry torque value. Having said that, I'm super careful to put anti-seize lube on SS bolts. I've had enough gall and break off that I really try to prevent that. Probably that affects the torque value to some extent. But it doesn't have to be slathered on either. Just get the threads wet. But more often than not, those are also used where torque values aren't given or really needed.

[NAZ]

The true measure of fastener or “bolt” tightness is tension not torque. A bolt under tension is actually stretched and wants to return to its at rest state similar to how a spring is stretched and returns to the relaxed position when the force is removed. The proper amount of tension is important. It applies a clamp load to hold parts together, helps resist loosening, and helps resist high cycle fatigue to name just a few.

The most common way to tighten a bolt to achieve the proper tension is to measure the rotational torque as the bolt is tightened. Torque values are specified to achieve the proper tension and can be derived through computer modeling or the use of a Skidmore gauge that measures actual clamp loading. Torque is a relative way to achieve proper tension but is a very poor indicator as so many variables can significantly affect the torque/tension relationship. The coating or lack of coating, lubrication, the use of washers, the condition of the threads all can significantly change the amount of torque required to achieve the proper tension.

The use of lubricants can reduce the torque required by 20% or more and the type of lubricant makes a big difference. The Engineering ToolBox published a chart of lubricants and their effectiveness at reducing the torque on typical cap screws and according to their data SAE 40 weight oil showed a 31%-41% reduction, white grease 35%-45%, and graphite reduces the torque by 49%-55%. It’s easy to see how lubricating threads and using a torque spec for dry threads can lead to failure.

As we see from above, the coefficient of friction has a great effect on the torque/tension relationship. The condition of the threads can substantially decrease the tension applied at a given torque spec. From the first tightening the threads are damaged and if the fastener is reused, the damage is increased which also increases the coefficient of friction in an unpredictable way. This means that reusing nuts and bolts will have a significant affect on the torque/tension relationship. That’s why all the major bolt manufacturers suggest that critical fasteners not be reused. No that’s not just a ruse to sell more fasteners. The company Bolt Science specializes in bolting technology (an engineering company not a manufacturer). Their studies show that the reuse of fasteners have a significant impact on the torque/tension relationship and can reduce the tension as much as 50% on the 10^thretightening.

[sbhunter]

The 8” thickness requirement for mine was due to height. I did not mean to imply that was a standard. I have walked over countless construction sites as a firefighter in central Indiana in the last 20 years. The VAST majority of residential construction here is 4” at best. I will not argue with you Paul, because I respect you and your opinion way too much. My point was that anyone considering a 2 post lift needs to check their concrete first.

[edwardb]

The 8” thickness requirement for mine was due to height. I did not mean to imply that was a standard. I have walked over countless construction sites as a firefighter in central Indiana in the last 20 years. The VAST majority of residential construction here is 4” at best. I will not argue with you Paul, because I respect you and your opinion way too much. My point was that anyone considering a 2 post lift needs to check their concrete first.

No arguments here! Hope I didn't come across that way. Just sharing my experience. We're on the same page. Proper concrete is important. Follow the mfg's instructions and guidelines exactly.