Torsion bar used to support weight 1

[DownHillHero]

I'm trying utilize a torsion bar to support a load as well as facilitate a flipping motion. A similar set up of this can be seen in a product known as a flippac and to start I was trying to figure out how the torsion bar in the flippac is able to support so much load.

The dimensions of the supported section of the flippac are about 80"x64".

I believe the torsion bar is made out of some kind of 4140 steel that has been heat treated and oil quenched. The shear modulus G=11600 ksi and yield tensile is 251600 psi. Since we will be dealing with shear stress, the shear yield is about 50% of the tensile, so 125800 psi. I have incorporated a safety factor of 1.5 giving an allowable shear stress of 83867 psi.

Here are the dimensions and constants I have been using to determine the torque in the torsion bar.

Torsion bar length = 54"

So far I have been using the formula
phi = (T*L)/(J*G)
where phi = angle of twist (rad)
T = torque (in*lb)
L = length of bar (54 in)
J = (pi/2)*r^4 (in^4)
G = 11600 ksi

At the point of interest, the load is about 500 lb which is acting 40" away from the hinging point. This results in a moment of 20000 in*lb, therefore the torque in the torsion bar must be 20000 in*lb to support the weight (the flippac has two support beams in the front, however 95% of the load for two people, about 400 lb plus the weight of the lid is supported only by the torsion bar). The angle of twist in the torsion bar at this point is 90 degrees = 1.57079 rad.

Using the equation above we can find the require radius of the torsion bar to allow for this motion to happen. From my calculation I'm getting r = 0.4407"

Next I used the formula t_max = (T*r)/J to find the maximum shear (t = shear). In this case the maximum shear is 148718 psi.... well above the yielding shear strength of 83867 psi.

Does anyone have any ideas on how flippac does it??? I am stumped.

 

[hendersdc]

My guess would be that there is another mechanical support somewhere in the system. Otherwise when you remove the load (people and retaining straps) the thing would would snap shut like a mouse trap and there is no way they would make you pull that hard to open it. The torsion bar is probably only to facilitate easy closing of the system.

 

[dvd]

I guessing that there are cables in the tent (or the tent fabric itself acts as such) that go over the centerpole and act as tension members to support the deck.

 

[Walterke]

Can you make a sketch?

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[rb1957]

where's the bending stress ?

if you're using solid steel bar, weight might also be a factor !? (i'd suggest tube)

4140 seems to max out at 180 ksi (Ftu), which has a Fsu = 108 ksi.

i looked up "flippac" ... couldn't see what you were trying to say ... sketch pls

 

[DownHillHero]

Thanks hendersdc and dvd, I also feel that there must be another type of support such as a cable. From my own calculations I figure that if you had two cables then each one would only have to support about 120 lb. which is reasonable. Another reason why I think this is because the angle of twist with just the moment created by the mass (about 100 lb) is about 78 degrees. If flippac can really support all that weight your right the thing would snap back to it neutral position rather fast.

Rb1957 you are right, there may be a bending stress, however the bar is being run through the hinging device so the weight of the steel torsion bar will have no effect on the actual torque of the system however it may be a significant factor in determining how much shear stress the torsion bar can handle. I will have to check that.

I have attached a sketch to give you a better idea.

 

[DownHillHero]

here is also a very basic sketch of the system.

 

[dwallace1971]

Looking at pictures on the flppac web site, it appears that there is a vertical support that extends down from each corner of the platform to the hood of the vehicle to which the shelter is mounted.

"On the human scale, the laws of Newtonian Physics are non-negotiable"

 

[rb1957]

as i understand your sketch, the torsion bar is out-of-plane, you're applying a bunch of torque. i assume the two support fttgs from the lid are cross-bolted into the torsion bar (so the torsion load is applied at two places). where is the torsion bar grounded ? (where is it's reaction to the applied torque?) i think you'll have travel stops and some latching (to keep the lid in the right place).

 

[DownHillHero]

There are the supports, however looking at the way they are designed and the way that they attach to the hood, I would not expect them to support a very significant amount of weight without buckling or breaking the hood mount.

From doing research the supports on the front are more to hold the flippac down when no load is in the lid section as well as preventing the thing from shaking in the wind, however they do not seem to be a significant load bearing support.

 

[rb1957]

ok the fttgs around the torque tube are guides, but the lid is attached to the torque bar by two fttgs (how does the torque get out of the lid ?).

and then the torque bar is attached to something (the rest of the world) since it has to take the torque from the lid and put it somewhere.

 

[DownHillHero]

To try to answer your question rb1957 your right the torsion bar is our-of-plane and a lot of torque is being applied.

The way it works is that the passenger side of the torsion bar is the point where is it grounded. This is because is if fixed to the part of the hinge that is attached to the canopy part of the flippac. On the drivers side of the torsion bar, the bar is fixed to the lid. They two other support have some type of bushing that lets the bar rotate. So this mean the passenger side of the torsion bar does not rotate, but the drivers side does.

Another thing to consider is that when the torsion bar is just supporting the weight of the lid (about 100 lb) the angle of twist is 1.359 rad which is about 78 degrees. The lid supports are then used to hold the flippac at 90 degree when only the weight of the lid is effecting the torsion bar.

Here is a sketch of what I mean.

 

[DownHillHero]

Going further to try to answer your questions the hinges that the torsion bar are attached to are embedded into the fiberglass and foam lid structure. There is a channel of some type of metal that they are attached to and they extrend about 8 inches into the lid.

To get a better idea of what this looks like here is a piture of a broken flippac. As you can see distributing the load from the torsion bar can be quite a problem.

 

[rb1957]

ok, the lid is attached to the torsion bar at one fttg, and grounded at the other.

i wonder if the structure is at rest (unloaded) with the lid down, and you have to work (pull on some cable) to close the thing ? does it "spring" open or closed ?

there's going to be some "honking" big fastener reacting the torque (with a couple only the diameter of the bar apart) ?

 

[DownHillHero]

Yes I believe that is how the torsion bar works.

When the torsion bar is in it neutral position (angle of twist of zero degrees), the lid is straight up. This way the torsion bar twists from a range of 90 degrees to -90 degrees (not ideal for torsion bars... but thats why we include safety factors!!) the idea is that this prevents the lid from slamming down into the top of your truck as well as slamming down when you are closing the lid.

To help open and close the system a two foot long rod is attached to the free end of the torsion bar. The operator can then get the needed leverage with the help of the torsion bar to open and close the lid easily. When completely closed or open releasing the latched or hood support rods should cause the lid to spring open and the far end should not raise higher than 18 inches.

To get a better idea I suggest watching this youtube video

http://www.youtube.com/watch?v=ydQ8S-1X64s

I also attached more drawings!

And yes after figuring out this torsion bar the next step is designing fastener. I agree they will have to be quite beefy!

 

[rb1957]

i don't think the weight is doing the twisting, i think the operator is twisting the torsion bar, bringing the lid down. weight is helping.

you're using a lid loaded with two people as the loadcase (creating the torque in the bar); but aren't there stops that the lid is resting on (in the down position) ?

if the operator is pulling the the lid down, then it'll spring back (once he releases his pull). i think the torque in the bar depends on the load that the "stay down" latches carry. i suspect that the "payload" of the lid (the 400 lbs) is reacted on the stops, and the torque in the bar is alot less than you've calc'd. i suspect that the latches load up about double the weight of the lid (ie the torque due to the latch load is about double the torque due to the weight), so that when the latches are releasd there is enough energy in the torsion bar to pull the weight up.

 

[DownHillHero]

From the video that I linked earlier once the operator passes the neutral point of the torsion bar he appears to apply force the opposite direction indicating that the weight of the lid is causing the angle of the twist and the operator is ensuring that this doesn't happen too quickly (if the lid is allowed to free fall with the mass we would have to consider the dynamic of the situation and in that case if the lid is allowed to free fall it might come into contact with the top of the truck).


I do think your on to something with the stops in the down position, however I looking at different pictures of flippac I can not seem to figure out where the stops are located (could possibly be a cable system running through the tent, once the cable was in full tension it could act as a stop).

The torque on the bar that I calculated earlier was based on the torsion bar being the only force supporting the weight and load. I have changed my analyse of considering the torsion to only facilitate the opening and closing of the lid rather than supporting any load (this is what the flippac really appears to be doing).

I have reworked the problem changing a couple things such as the length of the torsion bar (now 62") I will post an image to show my work. Results are giving a maximum shear of 93168 psi at 90 degrees which gives a safety factor of 1.35 rather than 1.5.

 

[DownHillHero]

Also to demonstrate what I mean by the mass causing the lid to fall I have created a matlab graph showing the moment created by the mass at the angle of twist increases as well as the torque.

The blue line represents the torque in the torsion bar and the green line represents the moment created by the mass. As you can see the two line intersect. At this point the torque in the bar is greater than the moment caused by the mass, therefore the mass lid will stop at this point.

 

[rb1957]

if the weight is causing the the twist of the bar, how does it twist up again ? (the weight force is never removed)

if the lid starts vertically, and is pulled down and held down, then released; it will spring up because the force to pull it down has been removed.

no?

 

[DownHillHero]

So the weight of the lid will cause twist of the bar to a certain point, for this purpose I set this limit at about 1.4776 radians. Once the torsion bar twists to this point the torque in the torsion bar will become greater than the moment created by the weight of the lid and the lid should be static at this point. The operator will then have to pull the lid down the rest of the way to and latch the lid down to achieve the rest of the 1.57079 radians of twist.

Now when then latches are released the torque is greater than the moment created by the weight of the lid so the torsion bar will spring the lid back up to 1.4776 radian (the point where the moment created by the weight equals the torque in the torsion bar). At this point the operator then attaches the lever arm and applies force to lift the lid up to the neutral point. Although the moment created by the mass is greater than the torque, there will still be torque in the bar which is creating an assisted lift system.

Does this make more sense now?