Reply With QuoteAh, ok here's the required dimensions. Please let me know if there's anything else. Bart
Bart, I'm sorry but I am still having a problem communicating to you the dimensions I would prefer to have, so please copy the below sketch and fill in the correct values on the shown dimension lines and then return it to me. The actual linkage lengths are unimportant, it is only the distances between the pivot points that matter. When you see the force diagrams you will understand this better.
Component Dimension Drwg.jpg
Ah, ok here's the required dimensions. Please let me know if there's anything else. Bart
Bart, I have analyzed the cylinder force required for the 20,000 lb and 7000 lb lift cases. The 20,000 lb case requires a minimuum cylinder force of 28,700 lbs at the 8.5 ft lift height for each corner of the lift; and, the 7000 lb case requires a minimum cylinder force of 28,700 lbs at the 12 ft lift height for each corner of the lift. In doing these calculations I have rounded off the dimensions and used a dimension of 105 in. on both arms to make the analysis trigonometery a bit simpler but these variances should not significantly change the results. I do not have time to do the calculation for the 20,000 lb force at the bottom point but based upon looking a the geometry angle I suspect the loading on this case is going to be somewhat less than that at the 20,000 lb full lift positon. Nevertheless, this calculation needs to be done to insure that. Just keep in mind that these loads are minimums based on an even distribution of the loads across all four corners and have no safety factor added. Realistically, you should at a minmum double these calculated loads for safety.
Attached below are force calculation schematics for your reference. If you have any questions just post them and I review them when I get back on line in a few days.
20000 lb lift analysis.jpg7000 lb Lift Analysis.jpg
Hello, looks like I'm getting into the habit of thanking you. Looks great and just one question as I think you meant to state at the 8.5' it'll take 57k as on your schematic?
I'm really curious how much repotting of the pivot points will reduce the force needed. I will stare at the model some more and see what more I can do with my limited experience in this.
Have a safe trip and I hope your weather is good. Thanks again Bart
Not to derail your thinking, have you ever seen a cable driven boat lift mechanism?
boat-lift.jpg
Yes, I sure have. Found some direct drive units 7000lbs at 1000$ so 4 ea would be pretty inexpensive really. Thing is with a boat
Lift, If you do have a failure the water is a bit softer than concrete. Thanks for the input and also the formulas on the retracting loss. Thinking a lot about what JAlberts said about my lift verse floor mounted lift safety. Floor mounted unit would fail at its lowest point logically. My design would fail at its highest point. I'm going to try and do better. You guys are a terrific help. Bart
Regardless of the design you will need to consider a safety lock-out as I think you will be walking underneath the elevated vehicle.Originally Posted by Bart
Right and the more a look at the scissor designs the better they look. Lots of pieces though. But they are a lot lighter for me to machine. I found some scissor calculations where you pointed me to for the hydraulic retraction rates. They show the cylinder running perpendicular to the load though. I have mine at angle like all the scissors I've seen. I don't know how much I will lose though. Looks like a lot less load for the cylinder and arms though. I have the starting angle of the cylinder at 23 degrees. Do you see any way to make it more efficient? Thanks for your help and advice Bart
I think with the actuator at the angle shown the mechanical advantage is minimal on the scissor lift design - do the math. At fully extended (vertical) the scissor lift actuator force is equal to the weight of the car plus the scissor frame.
Hello, unfortunately math is not my strong suit and not sure what you mean about the mechanical advantage being minimal. But I think I'm starting to understand then scissor design fully extended there's not much loss. It will be mostly due to friction I'm guessing. So, arm length is not an issue like my first design? The scissors mirrored arms balance the force?
Hello Bart, I had two great days of hiking and photographing in the canyons and am now back online for couple of days at least.
You are obviously correct about my 20,000 lb case cylinder load error, I am afraid I got into a bit of a hurry in trying to get my results to you before I left town.
I have looked at your above scissor jack design diagrams and this arrangement would provide the maximum cylinder loading at the beginning of the lift rather than at the top end. With that in mind, how would you like to proceed from this point?
Hello, great to hear your trip was pleasant and terrific to see your still interested in this project. Your calculations show the loads are just to extreme to be practical on my misguided design. Whileyou were away I've been looking at other avenues.
The scissor lift components will be easier to assemble as everything's down low and fairly light. Interesting thing is that the vertical post may not be required and be a potential source for jamming. It would be a good place for added safety measures though. It should also reduce sway with my compact scissor design.
My friend who once was in the hydraulics field suggested using Pilot Operated Check Valves attached to the cylinders as they only operate while there's pressure in the line. So if a hose bursts or the pump fails the fluid remains locked in the cylinder.
Anyway, I'm worked up several different scissor designs and will be starting the cylinder at 20 degrees. Here are some pictures of the latest design. I'm hoping you'll still have the time to help me with the math and offer advice on this design. I'm afraid you'll have to walk me through this one too. Any suggestions will be greatly appreciated.
Bart, I have taken a few minutes to review your above latest scissor lift design and in this design you show two horizontal beams on the lift platform. What I need to know is which of these represents the actual platform level, ie is it the bottom or top beam?
Hello, thanks for reviewing this for me. It will be the lower beam. I did this is to allow for more room for the stack up of the scissors and better angle on the cylinder. The upper beam will also stiffen the platform sides. They shouldn't interfere with the doors on the vehicles as one side will have the boat and trailer and I'll back the pickup on the opposite side and exit between the two. Should work the same for maintenance too.....I hope.
Bart, in looking at the drawings again this morning I realized that there is a set of dimensions I need in order to make my analyses. Please provide me with the dimension indicated below for all three of your lift height positions.
Scissor Lift - Dimen Request.jpg
Hello, yes those dimensions are 36.34",133.15" and 180.86" Thank you for your efforts. Bart
Bart, I have analyzed the scissor lift cylinder load for the initial bottom lift position, which is clearly the maximum cylinder loading in your new design; and, the cylinder loading at the start of lifting is 34,074 lbs. Attached below is my analysis diagram for your reference.
Scissor Cyl Load Analysis.jpg
Hello! Very nice! Well this is a dramatic improvement. Still a bit loaded on two of the pins though. I can see better what's happening but a also very surprised too.
If the cylinder were placed horizontal and attached to the arm where it slides would this be much more efficient maybe? Does the changing the arm length have as dramatic effect as changing the cylinder angle as far as cylinder loads are concerned? Thank you for all you have done. I'm trying to learn but even reversing your data I've yet to come up with the same results twice or even matching your data.
See last post too. So the scissor formula tool shows the cylinder horizontal but attached to middle off scissor. Would this arrangement be much different? Arms are the same except for the cylinder attachment holes were deleted as they are not needed.
Bart, I did an analysis before seeing your last post and my analysis is with the cylinder on the ground level but the result is the same as for your top level design. The resulting cylinder loading is 58,607 lbs. at the zero height initiation of the lift. See below attached analysis diagram.
Horizontal Cyl Analysis.jpg
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