What am I doing wrong?

[wbrian63]

Now that the construction of the shop is complete, we've got to do something with the scaffold we built to do the high work. The unit has a 48x72 platform and is 9 feet tall.

We broke it down into the two end frames, the X-braces and the platform. I found the weight-balance center of the assembly and attached two hook-eyes evenly spaced 3' on either side of the center.

Way up on the wall, I built two brackets to hold pulleys. On the right bracket, I tied off the rope, down to a pulley on the scaffold, back up to a pulley on the bracket, across to the left bracket and through a pulley down to the another pulley on the left hand side of the scaffold, back up to another pulley on the left bracket, and back down. I patterned this after the bicycle lifts seen forsale on the net:

http://www.amazon.com/dp/B001ENZFGQ/?tag=atomicindus08-20

Obviously this assembly is heavier than a bike - probably 250-300# total.

Once I get the lifting mechanics to work properly, I'll add a ledge on the wall to take the weight of the scaffold, and some safety brackets to keep it tight to the wall. Only using the rope and pulley setup to hoist it into place.

Problem? No matter how I tinker with the balance, it lifts a lot from the left and very little from the right. I can hold the rope and pull down on the left of the scaffold and it balances on the pulleys and stays there.

I've oiled the pulley sheaves, and the pulleys are all of the same diameter and capacity.

Why doesn't it lift evenly?

Here's a drawing of the mechanism. Ignore the position of the pulleys relative to the scaffold, and the non-vertical path of the rope - in use, everything is as it should be (I think).

Here are pictures of the rope and pulley placements - I'll refer to each segment as a point:

This is Point 1 - anchor, and Point 3 (where the rope comes back up from the scaffold and across to the left bracket)

This is Point 2 - attachment to the scaffold below the anchor (Point 1). Rope comes down, and back up to Point 3.

This is Point 4 - rope across from Point 3 and down to Point 5, and Point 6 - rope up from left attachment on scaffold and back down to me.

This is Point 5 - attachment to the scaffold below Point 4. Rope comes down from Point 4 and back up to Point 6.

Any ideas?

 

[srmofo]

I suspect with that amount of weight its going to pull the rope closest to you up first. which is the left side. Once the left makes it all the way the right will follow. Is this a problem?

 

[e-tek]

Here's my take: The pulley on the left is acting as the "moving" pulley, while the one on the right is acting as the "stationary" pulley. You need to pull from both sides to make both "moving "pulleys. Rather than an attachment point on the right, put another pulley there and pull on that side too.

 

[jake00]

I think the rope is stretching

 

[scott37300]

E-tek is right about the pulleys. The right side is acting like an "anchor" in sense. Because the left side has two pulleys at the top the second pulley has a lot of drag and thus the left side scaffold pulley will raise easier than the right side. Kind of tricky buy hopefully it makes sense, I"m not that good at explaining things!

My suggestion would be this, have one pulley on the left side and none on the right side supports. The right side will just be the anchor point, and leave the pulleys on the scaffold. Here is a ruff picture.

 

[ddawg16]

It's the rotating resistance of the pulleys.

What E-tek is describing is correct....but the reason it is happening is that the resistance of the downstream pulleys together is more than the first one or two pulleys.

To prove the point....have someone hold down the side that lifts first....this will cause the rope to pull through the other pulleys and start to pull up the other end.

About the only way you are going to get it to go up evenly is to pull both ends at the same time. Hence, install a couple more pulleys and attach both ends of the rope to the same pulling point or have someone pull on the other side....but in that case, you will most likely need someone to keep it straight as you pull up.

 

[wbrian63]

E-tek is right about the pulleys. The right side is acting like an "anchor" in sense. Because the left side has two pulleys at the top the second pulley has a lot of drag and thus the left side scaffold pulley will raise easier than the right side. Kind of tricky buy hopefully it makes sense, I"m not that good at explaining things!

My suggestion would be this, have one pulley on the left side and none on the right side supports. The right side will just be the anchor point, and leave the pulleys on the scaffold. Here is a ruff picture.

Thanks for the suggestion - I get where you're headed with this. However, your drawing effectively removes the block-and-tackle setup I've got, which is necessary to allow me to lift the entire bulk of this assembly (which probably approaches 250#) without drama or injury.

It's the rotating resistance of the pulleys.

What E-tek is describing is correct....but the reason it is happening is that the resistance of the downstream pulleys together is more than the first one or two pulleys.

To prove the point....have someone hold down the side that lifts first....this will cause the rope to pull through the other pulleys and start to pull up the other end.

About the only way you are going to get it to go up evenly is to pull both ends at the same time. Hence, install a couple more pulleys and attach both ends of the rope to the same pulling point or have someone pull on the other side....but in that case, you will most likely need someone to keep it straight as you pull up.

I've got enough spare rope that I think I'll just add another pulley on the right-hand bracket, remove the anchor point and feed it through that pulley. Then I'll add yet another pulley to the left hand bracket and feed the line from the new pulley on the right through and pull both lines at once. That should provide an even lift - rope getting pulled from both ends.

Maybe (probably) if I used high $ pulley's with ball-bearing or at least bronze sheaves, I'd not be facing this issue. However, this is a more-or-less one-time effort. Once the scaffold gets stored up on the wall - it's likely to stay there for quite some time. All I've got invested in this effort is the cost of the rope and a few screw eyes - the rest I had on-hand.

Thanks for the tips and replies.

 

[LEVE]

I'd try this to spread the weight/load over the entire frame. Cost would be minimal.

 

[larry_g]

Now that the construction of the shop is complete, we've got to do something with the scaffold we built to do the high work. The unit has a 48x72 platform and is 9 feet tall.

We broke it down into the two end frames, the X-braces and the platform. I found the weight-balance center of the assembly and attached two hook-eyes evenly spaced 3' on either side of the center.

Way up on the wall, I built two brackets to hold pulleys. On the right bracket, I tied off the rope, down to a pulley on the scaffold, back up to a pulley on the bracket, across to the left bracket and through a pulley down to the another pulley on the left hand side of the scaffold, back up to another pulley on the left bracket, and back down. I patterned this after the bicycle lifts seen forsale on the net:

http://www.amazon.com/dp/B001ENZFGQ/?tag=atomicindus08-20

Obviously this assembly is heavier than a bike - probably 250-300# total.

Once I get the lifting mechanics to work properly, I'll add a ledge on the wall to take the weight of the scaffold, and some safety brackets to keep it tight to the wall. Only using the rope and pulley setup to hoist it into place.

Problem? No matter how I tinker with the balance, it lifts a lot from the left and very little from the right. I can hold the rope and pull down on the left of the scaffold and it balances on the pulleys and stays there.

I've oiled the pulley sheaves, and the pulleys are all of the same diameter and capacity.

Why doesn't it lift evenly?

Here's a drawing of the mechanism. Ignore the position of the pulleys relative to the scaffold, and the non-vertical path of the rope - in use, everything is as it should be (I think).

Any ideas?

take the rope that is across the top and remove it from the pulley that drops it to the left pulley on the scaffold. Anchor the right rope on the left drop so that it looks like the right drop. Now take the horizontal rope and attach it to your windlass. So now you have two ropes to pull on and each side is the same, and independant of the other.

lg
no neat sig line

 

[buddyboy]

instead of using the anchor point, continue to run the rope back thru all the pulleys and pull on two ropes.

it should pull evenly and you'll have 3 stationary pulleys and 2 that move.

 

[gophergrove]

As Larry_g said, you need to have 2 lines to pull even. Anchor the left pulley line and pass the right line across to the pull point. Here are some pics of my luggage box rig. It's lowered partway in the pics so the pulleys can be seen. Works like a charm.

 

[bd8134]

I have the bike pulleys as in your link wbrian63. They do pull the bike up pretty even but then it is much lighter than the weight you are pulling. I bought the Garage Gator to store our bikes after I had my new garage built. This might be overkill for infrequent use, it is electric and can hold 220 lbs. This uses a winch with 2 metal chains with pulleys at each end. Maybe the heavier weights require the twin rope arrangement as others have mentioned.

 

[rickycobra]

Could it be the fact your using nylon rope? I would go with some other rope that won't stretch.

 

[istephen]

Think about rigging on a big crane. Ever see a beam attached to the hoist line in 2 places? not without 2 cranes.

Build a bridle to pick up the 2 points on the scaffold to keep the load level. Now, like on the crane, get your 2:1 advantage with your block and tackle above the bridle. Remember that the shorter your bridle legs, the stronger the bridle line has to be.

This way, you still pick up the 2 points on the load, and the separate anchor and top sheave means you still pick up 2 points you are lifting from so the loads are the same in the same 2 places. You can use the same number of sheaves you already have. You still get your 2:1 Plus, you only have to manage a single hoist line.

 

[wbrian63]

The rope's working load is 300lbs, so stretching isn't the problem.

The issue is the friction in the pulleys that's preventing the load from self-leveling. I can hold the rope and grab the scaffold on the left and pull it down to level it, and it stays where I put it.

I'm going to convert to a two-rope pull as suggested and I'm sure everything will be fine.

Regards

 

[srmofo]

what istephen said seems like a more logical solution to me when compared to trying to manage and lift 2 lines at the same time ....or like I said in my first post just lift it and level it by hand until it makes to the top.

 

[mbatarga]

I ran into the same issue when I made a pulley system to lift the hardtop off my Jeep CJ years ago. Pull on the one end of the rope almost to the point of the left side pulleys touching, if you can lift that far. Then go to the right side of the hanging scaffold and lift it to help it "balance" even - it should move up a fair distance. Then go back to the left side and repeat the process until it's lifted to the height you want.

 

[z28toz06]

3 pulleys vs. 2.
Put another pulley where the anchor point is now and place the anchor point on the board you are raising and lowering. that will equalize the distance traveled for the rope through both ends.