Part II, Forces

[edward lansing]

This thread will discuss the considerable forces in the hydraulic system. I am trying to give everyone a feel for what the cylinder is capable of when it gets moving. OK, stick with me for a moment.

1. For my 1702T the UVW (unloaded vehicle weight ) is 2690 lb.

2. How much does the top weigh? I guess about 40% or 1200 lbs. From jacking it up a few times in the ball park.

3. Each cable carries 1/4 of the total or 1200/4= 300lbs.

4. At the pulley head, each cable wraps around 180 degrees so the head sees a load of 600 lbs or 300*2=600 lbs.for each pulley.

5. There are 4 pulleys so the total load on the head needed to hold the top up is 600*4= 2400 lbs.

6. To actually raise the top there are friction forces, fluid pressure losses, etc. So lets say another 50%. The cylinder now has to put out about 2400 lbs * 1.5 or 3600 lbs. This is what a car or small truck weighs.

7. My cylinder is about 3" diameter or the piston has a dia of about 2 3/4 inches. This is an area of 6 sq in.

8. To put out a force of about 3600 lbs on 6 sq in you need a pressure from the pump of 600 psi. The book says the pump is good for up to 3000 psi so it looks OK so far.

9. The relief valve is set over 600 psi and I wish I could call the factory, let us guess that it is 1200psi give or take. Note that 1200 psi over 6 sq in is a force of 7200 lbs or 3.6 tons. A bunch.

10. If the relief valve is stuck or jammed the pump could put out 3000 psi. Over 6 sq in this is 18000 lbs or 9 tons. Think Godzilla under the trailer grinding things up!


Considerable forces. I can now see that if the cylinder gets jammed or stuck, there are considerable forces generated the could easily bend or distort guide rods and cylinders. In Part III I will try to explain what I found in my removal, repair process. How can these rods get so bent up? More later.

 

[JackandJanet]

Agreed, Jim. The pulley is not bearing a double load. On a single pulley, you lift 300# with a 300# pull, and the pulley support cannot see more than that 300#.
(Think what it sees if the pulley is locked, or if the load is "balanced" - just the force needed to lift the top).

But, I agree, make the appropriate changes and DO please continue!

- Jack

 

[edward lansing]

Part II Forces

OK, got the head scratching. Let's try this explanation.

Tie a rope to ceiling, pass it around a pulley, a pull of 300 lbs on the free end of rope will lift 600#. Rope tension is 300# and two supporting strands. Free body diagram says two pulls of 300# up and a 600# pull down.

Now attach the pulley to ceiling. A 300# pull will lift a 300# weight. But the pulley will load the ceiling with 600# load. Free body diagram says 300# weight down, 300# pull down, and 600# pull up on the pulley. I am searching web for verification. Lots of websites with "pulleys, free body diagrams, etc".

But we all agree that forces are considerable at 600# or 300#. More later, Part III will tell what I learned from my repair experience.

One more example: Take a pulley and attach to ceiling. Pass a single rope around it. First 300# man grabs onto rope and second 300# man grabs other side. Both men now suspended from pulley. Rope tension is 300#. Since system is stable the pulley pulls on ceiling 600#. If you wanted pulley to move up you have to pull up at 600# plus to get it moving up.

 

[JackandJanet]

You're right, now that I think about it Ed. The part that is confusing is that if you attach the "pulldown" cable to the 300# load, the force at the pulley is instantly cut in half! It's interesting though, none of the force diagrams in mechanics ever seem to consider the actual force on the pulley itself, only on the end of the rope.

- Jack

 

[JackandJanet]

Jim, another way to think of it (that finally occurred to what is left of my rapidly fading brain cells) is: Suppose you lift the 300# weight off a floor using the single pulley as described. Now, hang another 300# weight on the "pull-down" end and let go. Both 300# weights are now suspended above the floor by the pulley, which HAS to be supporting a 600# load.

- Jack

 

[sting32]

Jack and Ed, This is how I see it....................

Problem is you have diagram #3 with weight "off the ground", again there is 600 lbs on the pully, as it takes 300 downward pressure on one side to lift the weight. If you aren't off the ground everything is mute in the drawings.

 

[edward lansing]

In dwg # 3 there is a 300# tension in the anchored line. It holds up the weight of 300# weight.
Hence on the pulley there are two 300# forces acting down and there must be a 600# force holding everything up. Force wise, dwg #2 and #3 are identical for static stability. 600# needed to hold everything up.

Ask yourself, in Dwg #3 what happens if you cut the anchor line? It has 300# tension in it.

If you draw a so-called " free body diagram" around the pulley and list all the forces acting on the pulley, dwg #2 and Dwg # 3 are identical.

Boy, this has really turned into a great discussion. I am still working on Part III, coming soon.

I am sure we all are on the same page that the forces involved in this cylinder are large and can tear things up quickly if jams occur.

Not to muddy the waters, if you go the following website there is a pulley picture identical to the one we are discussing. The conclusion is that the pulley is held up by a force 2X the 300# weight ( anchored or hanging, same thing)

Website I found: Physics Forum > General Physics > Search for " Pulley, free body diagram " You have to register but it is simple.

Incidently this Physics Forum looks like it is constructed the same as the Hi-Lo forum.

Back to part III.

 

[JackandJanet]

Sorry Jim, it doesn't work that way. Drawing #3 is exactly the same as #2. You have to exert a 300# force on the pull rope (by the anchor on the floor, or by hand), to suspend the 300# weight on the other side. This creates two downward forces of 300# on the pulley which has to be countered by a 600# upward force to keep the system in equilibrium.

Here's another way to look at it that MAY help. Suspend the pulley from the ceiling with a spring scale. Pull downward on the rope with a force of 300#. There will be an equal 300# force at the other end of the rope where it is attached to the floor. Your "pull" handle will move down twice as far as the pulley and the spring scale will read 600# due to the 2:1 mechanical advantage given to you by the pulley.

As I said, we normally don't even consider the forces on a "fixed" pulley, so this creates the confusion - at least it did so in my feeble little brain.

But, please continue Ed. Sorry we sidetracked you.

- Jack

 

[JackandJanet]

Man I hate it when I'm wrong! Guess that's why I was a fireman and not a physics teacher! Learn something new everyday! I'll be quiet now Ed. Please proceed. Jim

In my opinion, there's no shame in being wrong - I just hate it when I'm reminded that I'm stupid! (And, I really felt stupid after my first post here.)

Your post here is very good, because we really don't think too much about the force on the pulley anchor - and we should!

- Jack

 

[renoites]

I have been reading this thread closely since my cylinder is leaking so much that I can't raise my top anymore.
I have noted several times that a "relief valve" is in the system to control excess pressures.
Where is this valve located. Can be different for other models but I assume that it would be in line before the cylinder and after the pump. Or is it built into the pump? I have not been able to find a relief valve but maybe looking in the wrong place!
$300 for new cylinder is silly since the parts to repair shouldn't cost more than $50 for new seals and such.
J&R stated that HILO didn't rebuild only replaced with new cylinder, and J&R continues that tradition. It may be quicker, but cost is out of line
Thank you for the "Hercules" parts reference.
I will be rebuilding mine.
Stuck in Dallas with no camper!


TIA
steve

 

[JIM L-HILO]

The relief valve is located in the pump. When the top is raised and the seals make contact, pressure builds up to the relief setpoint and the relief valve opens and bypasses the fluid within the pump. This prevents damage to the cables, pulleys and rails. Do not adjust this unless you have the knowledge and pressure gauge to do this. If your cylinder is leaking, this is where you need to start. If you have a little knowledge and proper tools they are not hard to repair. As you said aprox $25 to $50 worth of o-rings and cup seal. Any good hydraulic shop, farm tractor or construction equipment repair shop can also repair it for you. They can also ID the O-rings and seals. If you need any more info or help, contact me.

 

[PopRichie77]

I believe the relief valve is in the pump, but there is a manual valve on the pump to lower the top when the battery is dead, if this is open the top won't go up.

 

[JIM L-HILO]

Renoites, You did not say if the cylinder is leaking fluid on the ground or if you think fluid is leaking by the piston and returning to the pump reservoir. PopRichie is right that there is also a manual lowering valve on the pump and it must be closed (clockwise as you look at it) for the top to raise.

 

[renoites]

thank you Jim and Pop
the fluid is leaking from the ram end of the cylinder to the ground.
Will hone the chamber.
I purged the hydraulic system today. Not going to do any work on it until I get home and can put it up on blocks.
steve

 

[renoites]

OK.
I am home and the hilo is up on jack stands.
I have read other write ups about this job and all stated that the trailer was in the up position.
Mine is in the down position, and can't be raised!
I will document with pics my process for others that may need this info.
Can this be accomplished in the down position?
Cleaned area at the car wash and will start tomorrow.
Is there a step by step detail info for this repair in the down position?
The ram is threaded onto the cylinder rod, what would be the best way to remove the ram?
It appears that I will need to remove the lift wires from the frame to get enough movement of the hyd cylinder.
Any and all help regarding this project would be appreciated.
steve

 

[JackandJanet]

Steve - you should be able to lift the top manually with four "farm jacks". You can buy these at Harbor Freight, among other places, but they will probably be cheapest there. Here's a link to ones that raise 48", which I think would be enough: Farm Jacks - 48" 3-1/2 Ton Farm Jack

You'd have to raise each corner a bit at a time. I don't know that you have to disconnect the lifting cables to do this, but it should be obvious once you start if you need to.

- Jack

 

[renoites]

The thought of spending $200 for Chinese junk jacks inspired me to try it my way.
I figured that after 28 years the cables needed tuning anyway so I removed the cables from the frame. Cables were then moved so as not to be in the work area.
Removal of the lift cylinder was easier than I expected. Total time 2 hours.
Reinstalling will take longer since I do need to adjust cables.
I could not remove the ram so the re-builder got that also. didn't want to score up the cylinder shaft.

I can see where it would be easier with the top up, but I didn't have that luxury. Used a small bottle jack and wood blocks to move cylinder shaft to extended position.

I did take pics of removal procedure but was very straight forward.
Emails to J&R repair concerning this have been unanswered.

steve

 

[RichR]

J&R Repair is pretty much up to their ears in work so don't be real concerned if they don't get right back to you. I don't think they are heavily tied to their computer and the internet.

 

[robobd]

don't give up on j & r, they are the most helpful and honest of all hi-lo experts, thats why they are so busy

 

[renoites]

JR did come thru for me. They are very friendly and knowledgeable.

OK.
Doing the hydraulic repair with the top down is not an issue as long as you remove the cables. Very straight forward.
Approx 1.5 hours to disassemble and 2 to reassemble and adjust
I used tape to mark which cable, front and rear, was in the top hole in the frame. VERY IMPORTANT since all cables are different in length
Just mentally remember your approx adjusting bolt reveal and adjust to there as your starting point during reassembly.
Clean and lube cable carrier wheels since you are there.
Any hydraulic repair place can do the seal repair. Mine works better than ever now. Drain/purge all of your old fluid and replace with new auto trans fluid.
I did need to replace one cable, frayed not cut. JR knew the exact length of the cable I needed and furnished quickly and at reasonable cost.

This is a 3 on a 5 scale difficulty repair. No major tools required, just dirty.
Trailer should be elevated 4-6 inches for easiest access. So be careful when underneath.
steve