Help with some engineering issues on a folding tail gate ramp?

I'm building a folding ramp to replace my tailgate on my truck. It's going to be used to load/unload a ztr lawn mower.
(It's been built primarily from discarded bed frames, with the addition of some expanded steel grating.)

Because these mowers are back heavy, and low to the ground, a longer ramp is needed than you need to load a conventional mower.

My solution is a spring loaded 8' ramp that is comprised of two 4' sections that fold down and out.

It functions, it fits, the mower goes up and down it ok.

The problem is that it is so heavy, that after it comes down about 2/3 of the travel, it is very difficult to control.

When it comes down hard, it puts tremendous stress on the center - where the two sections of ramp meet. It can be hard enough to break it.

I know my welding isn't the best, but that's really not the issue here. This is a design problem.
I'm going to post a few pictures, to better illustrate my situation, and to show some of the things I've tried to soften the blow.

The picture of the partially folded ramp was taken at the point where it begins to get difficult to lower without losing control.

So, to clarify, I need a way to lower and raise the ramp with more control, and or soften the blow towards the middle. 

The springs I'm using are 140lb garage door extension springs, the largest springs available at any big box home center.

I need to get this finished yesterday, so I'm going to try some things while I'm waiting for a response, like move the springs back further in the bed to put more tension on the whole thing.
I'll also post this to the welding web forum, but I started here because I've got a lot of faith in the ingenuity of the instructables community.

If we get the bugs out of this thing and anyone wants it in instructable form, I'll do my best.

Thanks for reading the War and Peace of ramp questions.

-Amir

Comments

[s]

The vertical component of the force from the springs is small compared to the horizontal component. I suggest raising the points where the springs attach to the truck by around a foot. Not knowing the spring constant I can only approximate on the adjustment.

[Johnny Topside]

To stop the ramp from breacking in the middel I would have to small metal pieces on either side of the ramp and when you unfold it you slide a piece of steel in the slot and it will help control and the strength.

[broeand]

The hinge in the middle of the ramp could fold the other direction.  Make unfolding the ramp a two step process.  Unfold the two sections from the bed first.  Then unfold the last section to the ground.

[Amir]

That's just what I ended up doing, and it's been functioning smoothly for weeks.

I can't thank everyone enough. There have been so many  well thought out answers that I don't think it would be fair to select just one as the best. If the good people at Instructables, would let me pick a dozen, I'd try to narrow it down.

For the springs, I ended up with a neater, safer, design, based on the gorilla lift patent application.

Here's some pictures of the whole thing in it's current state. It just needs a sign.
Thanks to all of your help, I got it finished, and it got me my first paid welding job, which is now also finished.

Many many thanks again,

Amir

[torquelink]

my idea of this would be:
 
a slide in drawer version, just build in an hollow  rectangle space box in the bottom of your truck,  so u can slide the plates (which are on top of eachother  and have a slotted mechanism)

[Quercus austrina]

The problem has been identified. Initially by NachoMahma, on up. The angle of the spring is the problem here. Think of it this way, the shallower the angle, the focus of torque is further away from the work end of the spring. The most efficient is 90 deg. to the attachment point up to about 45 deg. After that, the spring becomes effectively weaker as the torque multiplies exponentially, moving the apparent attachment point away from the true attachment point. At some point, the spring cannot overcome the force and becomes invisible in the system. The best way to mod your setup is to redesign it so the angle stays in the 90-45 deg. arc. Try relocating the springs to the bottom of the bed at the sides, attatched to the front. then connect some cable to the loose end and feed it through 2 pulleys, one near the bottom of the bed and a second mounted approximately 2 feet above the top of the side. Then connect the cable to your original mounting point on the ramp. That should solve most all of your problems, including making it more safe. I also like the idea of putting some wheels or rollers on the end of the ramp.

Qa

 

REPLY

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[Quercus austrina]

Here's a rough drawing of what I mean: Picture. Use it if you want, modify it if you see fit. You could even enclose the spring to make it even more safe. I would use a pulley with a cable guide (looks like a U-strap over the pulley) for the lower pulley and then put a cable clamp on the cable near that pulley (between the pulley and spring) when the ramp is fully extended so that IF the spring were to break, the cable would be stopped from flying around and hurting someone.

Let us know how you solved this problem.

Qa

[jds311]

If you were to attach our spring to the front of the bed and then put a sheve (roller in a yoke) to the other end of the spring.   You will need to mount a frame to the end of the bed about the height of the ramp about four feet high.  Then attach the cable to this frame then run it through the sheve then to another roller mounted just above the cable mount.   Now run your cable up to another roller at the top of the frame and now to your ramp.  You will find this well give you more control and you well need to adjust the spring as it may be to strong.

[pbates123]

AnotherBrian has the right idea replace the hinges with, or just add,  torsion springs,  like you find on a roll-up garage door only smaller. If the torsion springs are strong enough you can eliminate the long garage door springs.

I would add torsion springs to the center fold as well. Also it's a good idea to make the springs replaceable. If they break you will be able to repair them.

So far it looks pretty good!

[captmike]

If you connect a wire rope to the point near where the spring is connected to the tail gate with a length connected to a stantion connected over the weight baring legwith another wire rope connected to the point at the eyebolt as the ramp extends it should complete the process.
[e-mail]

[AnotherBrian]

A torsion spring located in the hinge between the ramp and truck bed (replacing the two extension springs) would be the perfect solution here. It's resistance would increase as the ramp is pulled down which sould correctly balance the increasing torque.

[Cad-do]

I've designed one of these for 40K# equipment but to make it more user friendly I split it into 2 ramps.  Also, the use of steel makes the system naturally heavy. 
The 2 ramps and a nitrogen damper cylinder should at least help

[Midgetable]

stand on the tailgate, grab the middle of the springs and pull them up in the air! allthough i cant see where youve mounted the ends of the springs you need to raise the height of the 'car end' of the springs by about 1 foot!
its that simple! (the radius of the hinge relative to the mounting point for the car end of the springs has too small of a diameter, increasing the diameter of the mechanism by raising the mounting points will give you tention on the spring for a longer degree of travel on the ramp. if i was doing it ide cantilever the end of the ramp too like on one of those fold out toolboxes which would bring the weight of the end of the ramp closer to the top of it, see crude pic)
BUT...thats how to fix that idea when you could just use a winch instead!

[Zilduli]

 I will agree with the winch idea previously stated. It will allow for much more control of the lowering of your ramp. If you wanted to speed things up a little bit, perhaps you could attach a bicycle disk brake or two to the shaft of the winch and let the winch freewheel while regulating its speed with the brake. I'm not sure what kind of force/weight you are dealing with so more than one brake caliper might be necessary but something along those lines seems feasible to me.

[olguy]

I have a small trailer with a folding tailgate and agree that it can be unwieldy.
This  is how I do it.  I raise the back section to a verticle position and lower it as one unit.  Your gate may be too heavy to do this but it works for me.  You may also want to align and weld some pieces of pipe to each side of the gate near the hinge point to drop a pin (rod) through.  This may also strenghthen the gate.  Be careful not to pinch your fingers.   

[artistformerlyknownasjason]

Well I don't know if this would help, but the equation of a spring constant is F=-kx, F=force, k= constant, x=distance (0 distance is when the spring is at rest, don't worry about the negative). If you attach the spring to the ceiling and attach a mass the spring will stretch (obviously). Multiply the mass by gravity and you will have the Force F=mg (gravity =32 f/s^2 or 16m/s^2 in metric). F/x =k and you will have the spring constant. Average multiple Forces and distances to find a precise constant. If you know the weight of the gate (I would measure it in kilograms to make it easier ie 2.2lbs=1kg) The reason I explain this is because you will know how much force is being applied when the spring is @ a certain distance. The gait is using a torque  that will change with the angle of gait. Technically torque=Frsin(angle), but I would take force*radius of the gait*cos(angle of the gait). 90 degrees would be straight up. cos(90)= zero. It would also be easier to make the gait one piece because you would have to figure the math multiple times. Well good luck. I know this is a long complicated answer, but you will find most of this on Wikipedia. Good luck

[Re-design]

All these great answers and still not a best one of the bunch!

[glenm]

the winch is probably better but for an alternate solution. if you know how to configure pullies you could use pullies and some parachord or strong woven rope that is easy on the hands. two pullies would cut the weight in half and be faster than winching. you could make it so that your in the box of the truck out of the way while lowering it. or off to the side of the truck.

[sparkymsp]

I strongly agree with Paganwonder, get some safety cables on those springs or someone is going to get seriously injured for life.  I've seen pics of dudes with half their face ripped off. 
You are not the first one to try this route.  Everyone elses answer was hydraulics.
Or, turn your ramp into a "Tiliting" trailer, as much time as you spent you are already half way there you just need some wheels.
Good Luck. 

[acidbass]

 umm i am thinking istead of springs use the old fashioned steel cable and i think tension is your problem

[paganwonder]

I would go with a boat trailer winch and cable.   And for the sake of everyones' safety please run a safety cable thru that spring!  If it breaks loose there will be hell, pain and anguish to pay!

[Re-design]

On the first picture there is a post stick up in the side of the bed.  Lengthen this post to the length of the ramp folded up.  Attach a pulley to the top and a boat winch to the bottom of the post.  Not attach a cable thru the pulley and hook it to the hinge point of the ramp.  Now you can control the lowering and raising of the ramp


I'd leave the springs but you need to run a cable thru them like on a garage door since they are streatched and if they break and they do often they are a huge hazard.

[steveastrouk]

+1 I didn't spot the posts. Good plan.

[caarntedd]

+1 Great plan, especially the cable within the springs to prevent taking your head off in the event of failure. What about springs or gas struts under the hinge point to assist with smooth opening/closing in conjunction with the winch?

[jeff-o]

I suggest replacing the springs with cables controlled by a winch.  That way, you'd have a controlled "unfolding" for the entire process.  Oviously, make sure you get a winch that is up to the job!

[lemonie]

How about not having this fold? Just have a two-piece ramp that fits together.

L

[steveastrouk]

Looking at this, the main problem is the springs need to do most work at the point you are at now, but they are becoming much LESS effective, because they are acting at a very low angle to the ramp.

[kelseymh]

What Steve and Nacho said :-)  Would something as simple as replacing the chains with springs, between the midline support and the lower half, do the trick?

Nacho's suggestion of putting wheels on the ground is excellent, that would let the ground take the load at the far endpoint, and give you some control in pulling the bottom outward.

[steveastrouk]

You look like you'll need springs on the lower section too.

[NachoMahma]

.  As the rear section swings out, the amount of leverage you gain is overwhelming your springs.
.  Add some rollers to the back edge of the ramp (the edge that touches the ground. Use size/shape appropriate for surface you will be using ramp on. Adjust everything so the rear edge contacts the ground a little before the shown position.
.  May still need to add springs and/or shock absorbers/dampers between the two pieces of ramp.
.  All that's just off the top of my head. I may have missed an important detail or two.