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Old 09-23-2014, 09:09 AM
ShawnR ShawnR is offline
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Default scissor lift calculations

Hi all

I would like to build a small dump trailer, up to 2000#. I have been considering using a 3000# winch and a scissor mechanism. I found this site and was looking for someone to confirm my calculations urrrr, number inputting...;-)

http://www.engineersedge.com/calcula...ter-loaded.htm

If I understand correctly, I would be best to have a 45 degree angle on the lift arms when the bed is flat, then after that, the lift continues to get easier. Is this correct?

Has anyone built this? I figure a winch and battery is easier than hydraulics for a small trailer behind a quad.
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Old 09-23-2014, 04:24 PM
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Scotts Scotts is offline
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Well, I do not know how or what to tell you about your project. I can do my best to be polite and say Welcome in and have another cup of whatever you prefer and have a little patience we are having a monumental case of reading room syndrome right now. It goes with the weather and football season.

Thanks

Scott
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Old 09-23-2014, 08:27 PM
o7oBaseMetal o7oBaseMetal is offline
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I am not necessarily looking to build one but I use alot of scissor lifts out on jobsites. I am not sure what angle they fold down to when all the way down but it is not much. It's kinda crazy how much force it takes compared to how heavy the actual load is. Of course the stroke is quite a bit smaller than the lift travels up so that must be what makes it practical. Very interesting topic.
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Old 09-23-2014, 11:12 PM
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RancherBill RancherBill is offline
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I can see where you're going with this. My thought is it's more complicated and the welding and material has to be better because of the force.

Here's some low tech winch dump trailer that are easier to build.

http://i256.photobucket.com/albums/h...wler/110-1.jpg
http://i.ytimg.com/vi/QPxhIMvKwJ8/maxresdefault.jpg

And the one I thin is ingenious is attached.

All of the designs require a chain to prevent over tipping.
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Last edited by RancherBill; 09-23-2014 at 11:18 PM.
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Old 09-24-2014, 06:56 AM
ShawnR ShawnR is offline
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Thanks for the greeting Scott. I think I have visited here so many times, I did not realize I had not posted before. It is a great site for shop guys.

o7o....I have never even used a scissor lift but find the mechanism interesting. I just started researching it for this project.

Bill, the one you attached is more like what I had planned. I guess I showed a full scissor lift but was thinking "half scissor" mechanism. The math on that website shows that if the included angle gets less than 45 degrees, then the pulling force increases dramatically so maybe I should rethink the project. I could go smaller like a lawn tractor size trailer and then position the wheels so that I get some counterweight. My hesitation here is that once it starts tipping, it "flips" suddenly. These new linear actuators look interesting but I wonder what the holding force is, ie if a trailer load flips, can the lift actuator take the force in that direction. A larger trailer (back to the 2000# one) would probably justify a hydraulic lift. No rush. As many of you, wrapping up other projects this fall. Gotta finish some before starting another....
Cheers,
Shawn
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Old 09-26-2014, 01:04 PM
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Big_Eddy Big_Eddy is offline
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See attached "PowerPen" . It should give you an understanding of the math for a scissor lift dump trailer.

The largest force occurs when the bed is flat, and the scissor is least open. If you can get it moving, the forces drop from there.

The scissor lifting force drops quickly once you get under 45 degrees open (20% of winch force down to zero).

The further the scissor is from the bed pivot, the lower the winch force needed, but the bigger the scissor legs need to be (and therefore the smaller the closed angle will be when you squeeze it into the 6" or 8" or .. gap under the bed ) to get to a 45 degree dump angle.

You can reduce the forces needed by moving the bed pivot ahead up to 25% of the bed length (overhang), and if you use a pulley block on the winch cable, you double its effective output.

If you want to sketch a design with some dimensions on it, I can do the math.
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Old 09-26-2014, 03:09 PM
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RancherBill RancherBill is offline
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Eddy has all the math and the initial lifting require the most force.

The pic I attached sort of addresses the issue. If you look at it in the closed position. It is a hybrid design - scissor lift and 1/2 straight pulley lift.

The tower where the winch sits actually will lift the front of the trailer as it is trying to get the scissor attachment point to move. This will reduce the power required.

Yes, it will go quicker as the load goes higher. It should not flip because the scissor itself will prevent it over tipping.
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Old 09-26-2014, 05:09 PM
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The design of Bill's sketch is ingenious. Full flat bed when the scissor is ineffective, the winch cable is pulling up on the front of the bed with full lifting force at the furthest point possible from the pivot. As the bed starts to lift and the scissor starts to open, the winch force then starts to pull directly on the scissor. It solves the inherent weakness with the scissor design when closed.

When Bill mentions overtipping, what he means is that there MUST be a positive stop on the scissor lift for any cable operated system. Otherwise if it goes to past 180 degrees the force starts pushing on the string and the bed will crash down. A chain, a stop, anything that will prevent the scissor opening beyond 180.

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