Vehicle lift

[Bart]

Here's the lift I'm trying to design, and the parameters are as follows.
Lift 20,000lbs 8.5' for winter and park cars underneath.
12.5' clearance underneath when not in use.
Lift 7000lbs at 12' from underneath with nothing on top.
Use as a general purpose hoist.
Use for vehicle and trailer maintenance.

The hydraulic calculation tables are a bit too much for me I'm afraid. Any suggestions would be appreciated. There isn't a commercial lift that meets my needs. I'm worried I have too severe of an angle on the cylinders. I'll put pins in for safety.
Thanks Bart.

[GlennD]

I highly recommend that you drop by a local tractor truck repair facility and have a look at what they use to hold a dump truck in the air. Then you might consider to store one in the air you could just about double the strength they have to hold them up even higher than they do while working underneath for a few hours. Also consider the ground under it has to be part of the design, and as you say, a backup safety lock mechanism like on an elevator to prevent a sudden drop if a hose fails. Also with that kind of pressure an oil leak could sever limbs so thats something else to keep in mind.

[Bart]

I highly recommend that you drop by a local tractor truck repair facility and have a look at what they use to hold a dump truck in the air. Then you might consider to store one in the air you could just about double the strength they have to hold them up even higher than they do while working underneath for a few hours. Also consider the ground under it has to be part of the design, and as you say, a backup safety lock mechanism like on an elevator to prevent a sudden drop if a hose fails. Also with that kind of pressure an oil leak could sever limbs so thats something else to keep in mind.

Thanks Glenn, your right on target. Our architect has asked to see what I had in mind so I drew this up so he could reinforce the floor. All I've seen are cable units except for the old style hydraulic. We may be talking to you in the future as we allowing room for a people lift when we get older. Great help thanks again. Bart

[JAlberts]

Glenn, your displayed mechanism will definitely not do what you want because it cannot apply the required leverage throughout the lifting travel you require. Basically, your design is based upon a set of a toggle mechanisms and these only are effective when the force on the center is directly perpendicular to the direction of travel of the toggle arms. Even then there are definite limits to their operating range because although there is a large separating force when the base ends of the toggle far apart, when those ends are very close together there is essentially no separating force for the mechanism and it load lifting capability is essentially zero.

The reason the hydraulic lift and cable lift designs are used is that both can apply a constant lifting force regardless of the height of lifting platform and the lifting force is directly in line with the direction of travel of the lifting platform which is the most efficient way to lift the most weight with the least power required and the least extraneous loads and stresses on the lifting system structure.

[Bart]

Glenn, your displayed mechanism will definitely not do what you want because it cannot apply the required leverage throughout the lifting travel you require. Basically, your design is based upon a set of a toggle mechanisms and these only are effective when the force on the center is directly perpendicular to the direction of travel of the toggle arms. Even then there are definite limits to their operating range because although there is a large separating force when the base ends of the toggle far apart, when those ends are very close together there is essentially no separating force for the mechanism and it load lifting capability is essentially zero.

The reason the hydraulic lift and cable lift designs are used is that both can apply a constant lifting force regardless of the height of lifting platform and the lifting force is directly in line with the direction of travel of the lifting platform which is the most efficient way to lift the most weight with the least power required and the least extraneous loads and stresses on the lifting system structure.

Hi JAlberts. Thanks for your concerns. Yes, I agree that this is really pushing the reasonable limits of force. But I think it may still work. The cylinder angles are not to severe at the 8' hight where I need it to lift 20,000 pounds. All the way up I need it to lift 7000 and that may be an issue. This is a lot like a scissors lift. In fact Bendpack has a 7000 lb scissor lift on YouTube that running their cylinders as severe an angle as mine. I can easily get 32" cylinders 20,000 to 150,000 lbs each. Are you sure it still won't articulate with these capacities?

[Bart]

Glenn, your displayed mechanism will definitely not do what you want because it cannot apply the required leverage throughout the lifting travel you require. Basically, your design is based upon a set of a toggle mechanisms and these only are effective when the force on the center is directly perpendicular to the direction of travel of the toggle arms. Even then there are definite limits to their operating range because although there is a large separating force when the base ends of the toggle far apart, when those ends are very close together there is essentially no separating force for the mechanism and it load lifting capability is essentially zero.

The reason the hydraulic lift and cable lift designs are used is that both can apply a constant lifting force regardless of the height of lifting platform and the lifting force is directly in line with the direction of travel of the lifting platform which is the most efficient way to lift the most weight with the least power required and the least extraneous loads and stresses on the lifting system structure.

JAlberts,
Thanks for pointing out the faults, and I have tried to redesign with better results this time. Yes, a cable system would be the best route. However, I have new USA made 36" hydraulic cylinders rated at almost 30,000lbs ea. So, this is why I'm pursing this simpler to build scissor design. If need be I can double up the cylinders for a total of 8. Your assistance is very much appreciated. Bart

[JAlberts]

Bart, the real basic fact is that no type of scissor arrangement is going to satisfy your requirements because you require the same lifting force at all platform heights to get your load from ground level to your required lift heights and there are scissor type configurations that can do this.

With the above said, one bit of additional information I need before making any additional comments or suggestions is the stroke of each of your hydraulic cylinders.

[Bart]

Hello JAlberts, I'm sorry for my inability to design, clarify and understand the fundamentals of hydraulics.
The cylinder has 36" travel and with four will have the colum load of 120,000 lbs.
I need the lift to raise 20,000lbs to 8ft. Would it be possible to do that? Would it get to 12' with no load?
Bendpack has a 12,000lb scissor lift on YouTube. The lowered lift hight is only about a foot. Their cylinders are running pretty extreme angles.
Thanks so much for your assistance. Bart

[JAlberts]

Bart, I owe you an serious apology as to the possiblity of your lever designs. I visited the Bendback website you referenced to view their lifts and I admit I was a bit surprised at what I discovered. Using some rough, and I do mean really rough, scaling and calculations from stopped views on their 12,000 lb lift model video it would appear that with an about 3 to 1 lever ratio and a 20 degree lateral offset of the cylinder rod pin location relative to the pivot pins' centerline the loading on each of their six cylinders at the beginning of lifting would be about 23,000 lbs, which is achievable even if it does require some very strong cylinder and rod pin connections and lever arms to stand the imposed forces.

Based on this it appears that there is some possibility of using your last above concept for your lift. Even so, since your desired lift height is 12' compared to what appears to about 6+' for their unit there will need to be some balancing of lever ratio vs cylinder stroke vs cylinder load.

With respect to your different loadings at different heights, as opposed to the Bendback design, your cylinder and toggle loading will increase as the platform lifts to your different load height and load combinations so each one of these combinations will need to analyzed to determine which one provides the important load in the design of your lifting arms and cylinders.

Another issue with your design is that you are using the retracting action of the cylinders to lift your load and you need to be sure that the cylinder ratings you are using are for this action because the retracting load capacity of the cylinders is less than their rated extension loading due to the reduction in the piston pressure area being reduced by the rod area on that side of the piston.

An unfortunate safety issue in your arrangement as opposed to the Bendback design is that on their arrangement if there is any failure due the high stresses on their unit it will occur at the lowest point of their lift which give it an inherently safe design failure mode. With your arrangement the risk of failure rises as your platform heights increase which is not an inherently safe design failure mode. As a result, a safety ratchet feature similar to that on the Bendback unit that will immediately catch the lift if there is a failure during the lifts operation is almost mandatory for safety on your unit.

One additional note of caution on a issue in another forum thread posting today has to do with the simultaneous extension and retraction of all lifting cylinders during the lift operations. If you read the text on the Bendback 12000 lb. unit, they made a specific statement about the "sophisticated hydraulics of their control system". At least one item this is addressing is the fact that simply applying equal hydraulic pressure to multiple cylinders does not insure they will all extend or retract at a simultaneous rate. In your case, any imbalance of loading on your platform will result in the lightest loaded corner lifting until its column guide becomes jammed before the other column cylinders will start to lift. Then the rest of the corner cylinder will continue this action in their order of loading; and, this action also occurs in reverse order during cylinder venting while lowering the platform. Apparently Bendback is using some method in their system to insure that the rate of hydraulic flow to and from all corner cylinders is maintained to keep their lift level during operation and it is paramount that you insure this is incorporated into the hydraulic controls for your system as well.

If after my initial poor submissions on your project you still have any confidence in my input, then once you have some specific dimensions for your toggle design I will be to review them and give you my feedback but as you see from the above as we investigate the design of your lifting system more and more elements and complexities to be addressed are being revealed. For this reason, I really strongly recommend that if all possible you seek out an experienced manufacturer to design and fabricate your lifting system both for safety and because I believe it will ultimately be less expensive for you.

[Bart]

Hello JAlberts,
I'm honored that your spending your time looking at this odd project. Thank you also for looking at the YouTube video. The six cylinder system is very interesting and also that they load with the heaviest part of the vehicle above just the two cylinder portion. That safety ratchet is similar to ones they use on their cabled hydraulic units. In addition I will have safety pins that I will put in place manually.
Excellent point on the potential uneven lifting. That will be my next hurdle to overcome. Thinking of load equalizer or a monitoring system to assist control. It's been weighing on my mind but may be a mute point if the design is impractical. Good analysis on the Bendpack units chance of failure will be at its lowest point too.
I have an acquaintance that use to be in hydraulics and he's offered to assist with the hydraulic system once I have the design hammered out. He's also offered to have is old company double check everything for me.
I very much hope you still have time and interest is getting this lift to possibly function. Your insight has been invaluable and I would be going along blindly without it. Bart

[Bart]

Please see my above post. I can not find any specs on my dual action cylinder retracting capacity. I've yet to find any tables for retracting verse extending strength differentials. Is it usually a considerable amount?
I'm attaching some dimensions and hopefully I've captured enough to work from. I'd be happy to change my pivot points if anyone wants provide input.

[Kelly_Bramble]

Please see my above post. I can not find any specs on my dual action cylinder retracting capacity. I've yet to find any tables for retracting verse extending strength differentials. Is it usually a considerable amount?
I'm attaching some dimensions and hopefully I've captured enough to work from. I'd be happy to change my pivot points if anyone wants provide input.

Extend vs. retract forces are available from the hydraulic actuator manufacturer or can be calculated.

see:

Actuator Extend Force Calculator

Actuator Retract Force Calculator

[Kelly_Bramble]

Are you limited to this lifting design configuration? Thee are many other approaches that work great..

[Bart]

Hello,
Thanks for providing the links for the cylinders. They are very nice to work with.
I'm open to any suggestions for design. The drive on floor mounted units are out as I need the floor space underneath and need 12' clearance too. They make a 12' lift but not in dual configuration or near the capacity I need. The dual lift they offer is also to light and short.
I have 4 new USA prince 64036 cylinders and could get 4 more if I need to double them up. So, this is designed around them.
this lift is pretty easy for me to build with my older mill and lathes. Still lots of challenges to be met.
If anyone's interested I have some unusual designs on YouTube BartDavis100, you may want to see my cantilever walkway, articulating trailer hitch that's spring loaded, folding helm seat, boat cut in two for carrying, and a boat/car/utility trailer that's in fabrication now, but just a rendering on video for now. Anyway a lot of different stuff. Always fun to try something new.

[JAlberts]

Bart, I hope Kelly's referenced formula can answer your cylinder rating question; but, you can also simply use the area ratio shown to determine the retracting load rating based upon your known extension load without being concerned about the pressure. The equation is simply: F retracting = F extending X (A piston - A rod) / A piston.

Thanks for the drawings, I will take some time to look at them and try to give you some feedback; but in the mean time, I would like some additional information regarding your loading conditions and values. I was glad to see that you noted the same anomaly in the Bendback cylinder arrangement that confounds me a bit as well. It would seem that, if there is going to be more lifting force on one end than the other, then it would be logical to place the largest force on the vehicles heaviest front ,end not the back end. I really don't understand why they chose the alternative either; and, it does point out an issue to be addressed about the distribution of your platform loading when determining the maximum rating of your four corner cylinders. Initially, my thought was to evenly divide your specified loadings across all four cylinders; but, this will not be correct if the loads you will be placing on the platform might biased either front to back or side to side. Please give me your input on this loading distribution issue. Another critical question I have about your given loading values is whether or not they include the weight of the lifting platform itself.

With regard to the discussion about parallel lifting, you made a comment about "load equalizing", I suspect that it is only a matter of terminology but just to be clear, what is required is "flow equalization". The distribution of loading is going to be totally controlled by the way that you load the platform. In visualizing the concept, think of a four one cylinder pumps with their shafts connected and one pump connected to each of your hydraulic cylinders so that each hydraulic cylinder will receive the exact same volume of fluid and (assuming all cylinders are the same size) lift exactly the same distance on every rotational cycle of the pump set. I am sure that this clear to your friend, but I just want to make sure I am covering all bases as best I can in our discussions.

[Bart]

image.jpgimage.jpgimage.jpgHello, <br>
absolutely great stuff here. This is really amazing and a bit over my head. Thank you for your patience as I digest this all. <br>
I have added the platform weight into the total. Unfortunately, the platform will be loaded unevenly. 7700lb Diesel pick up and a 6000lb boat on a 2500lb trailer is what needs to be lifted. Best I can do is have opposing corners of the lift have the heaviest loads.<br>
Looks like retraction loss is quite high like 30%. So I lengthened the lower arm and put the pin hole the same distance away from the pivot point. Now we are pushing. I will need to move the upper arm hole closer as it's 2" over travel now.

[JAlberts]

OK, so now I need some basic dimensions on your change and hopefully I can explain them without pictures.

What I need is:
The new distance along the lower arm between:
1. the platform pin and the top arm pivot pin connection
2. the platform pin and the cylinder end pin location

My calculations show the original distance along the upper arm between the arm connections pin and the cylinder connection pin to be 57.72 in. so this distance on the new design is being reduced by 2 in. to 55.72 in. with the same 20 in. perpendicular offset from the arm centerline.

Note: For my calculations the pin to pin distances and the angles of the arms to the horizontal plane in each lift positon are what are really required for calculating the hydraulic cylinder loads. using what dimensions you have given I have been able to determine those dimensions and angles for the top and bottom platform positions in the prechange design already.

[Bart]

OK I hope I understand what you need here. I'm still mulling over your excellent explanation of flow with 4 pumps. Takes a long time for these ideas to sink in.

[JAlberts]

Bart, I have the new drawings, but to be honest it would save me a lot of time if you would dimension the arms and hole positions on the arms in the assembly drawings drawing in the manner you did for the overall length of the bottom arm and as shown marked in red on the below example. Since I know the lift heights of each position with these dimensions I can easily sketch single line drawings that allow me to use simple trigonometery to determine the force angles I need for resolving the different forces.

Just as a heads up, staring Wednesday morning I am going to be offline for a few days while I head to one of my favorite North Texas canyons for a couple of days of hiking. I should be back on line sometime this next weekend.

Dimensioning Example.jpg

[Bart]

Thanks so much for helping and also putting what you need in red on a sketch. I got those for you. I'm really curious what you think I accomplished with the switch to pushing or did I lose most of it with the increased length and with more leverage applied now? I got the new dimensions here for you. If I don't hear from have good safe trails! Bart