Pressure on purchase line

[porkchop]

So this came up at work and I had absolutely no answer for it so I thought I'd throw it out to CB. Does the amount of weight on the batten of a correctly weighted counter-weight system effect the amount of force on the purchase line?

From my admittedly limited time on the fly rail it seems like the weight overall weight did have some effect on the starting and stopping of line sets. I would guess that has something to so with inertia and momentum, but it's been a long time since I studied either in school.

ADDED LATER: Also if the total batten weight does effect the amount of force required to operate the purchase line does anyone know a formula or have a rule of thumb for how the two effect each other?

 

[techieman33]

Yes it does have an effect, the more weight the harder it is to move. I would imagine it's pretty linear in how much force it takes to move a heavier batten.

 

[Footer]

It really depends on how much friction is in the system. With more weight on a batten, you have more weight pulling on all the blocks. Therefore, the blocks run slower. In a double purchase system this is painfully obvious. Now, in a perfect world, something in balance should be able to move perfectly with even an ounce of force. With mechanical strain however thats not true. When you pull a purchase line, you are actually adding weight to the line to tip the balance in your direction. The more weight you add, the faster the set moves. I'm sure you could come up with a formula to figure out how much force it takes to move X @ Y speed. I have a feeling that the guys that design traction winches deal with that. However, because every system is different there is noting universal. Types of blocks, types of cable used, covering on the blocks, and all that kind of stuff can slow down a set.

 

[MPowers]

It is, as Kyle and techieman said, a direct increase. It is geometric rather than arithmetic however. There is no "formula" for how much. The number of lift lines, the diameter of the loft, tension and head blocks, single or double purchase, grid height, compensation chains, etc. all add to the possible variations. Inertia and system friction are the largest factor in the increasing amount of force needed to start the system moving and to keep it moving.

 

[porkchop]

Thanks guys, we rig with chain motors almost exclusively so it wasn't my cup of tea. Sounds like like might get really interesting when we play that house where they maintain really low batten capacities because of the purchase lines.

 

[Van]

Do not, my friends, confuse 'weight' with 'Mass'. Technically all the things mentioned so far are correct; an increase in friction on pulleys will increase the effort required to move the batten. But remember that even in weightlessness a given object has mass. The whole point of a counter weight system is to remove the effects of gravity and to overcome weight, but to overcome the weight of the loaded batten you, necessarily, double the amount of effort, energy or work that must go into the system to begin the movement of the system.

 

[FatherMurphy]

maintain really low batten capacities because of the purchase lines

Sounds like someone's past due to rerope the linesets. If they don't want to fly heavy objects normally for fear that the purchase line will break, what do they think will happen if something snags or gets shock loaded?

 

[fredthe]

Also if the total batten weight does effect the amount of force required to operate the purchase line does anyone know a formula or have a rule of thumb for how the two effect each other?

Basic Physics: Force = Mass x Acceleration.

Translated as the more mass in the system, the more force will be required to get it moving. Keep in mind that in a single-purchase system every pound that you add to the batten adds 2 lbs. to the system; for double purchase you are adding 3 lbs. (Using lbs. as mass)

I would submit that if increased friction under heavier load is contributing significantly to the force required to move the lineset, you've got much more serious problems.

-Fred