Double purchase and single purchase

[theroo]

hi!

 

can someone enlighten me on what the advantages and disadvantages of such systems are?

 

Thanks!

[andystone]

Depends on what you consider an advantage or disadvantage!

 

Single purchase means that the cradle travels from the stage floor up to the grid and you put equal weight on the cradle to the weight on the bar,

 

Double purchase means the cradle only travels from the loading gallery to the grid, so you have more space on the studio floor, but you have to put double the weight on the cradle for the weight on the bar, so you have more lifting to do and it can limit the amount of weight you can put on a bar unless the cradles are bigger and everything else is uprated.

 

Andy Stone

Stagetec Distribution

[Ben]

the advantage of a counterweight system are that very little effort is required to move the item bing flow as the whole system should be in bbalance its like a big set of scales where you weight the ccradlewith the same amount as on the bar (or the ppiecebeing flown is if not on a bar) and then by moving the ccradlethat is attached to the bar via ppulleysthe bar or ppiecewill move acaccordinglythe ccradlegoes down for the bar to go up and vice versa)

 

this is easy in single purchase as the lines just go from the bar to the grid, over one pupulleyalong the grid to one side over another pupulleynd then back down to the cradle, a line is then attached to the ccradlethat is just a loop that enables you to pull it in or out. the bar will move the same amount as the cradle and this will require you to have a counterweight system the full hight of the fly tower.

 

in double purchase this is complicated as the cradle only moves half the distance the distance the bar does, very useful when you need to have a fire exit or sosomethingn the wall that stops the counterweight cage brbeingble to reach the deck. double purchase woks prprimarilyhe same way as single purchase but with the addition of an extra set of ppulleysmeaning that the distance ttravelledby the bar is double the distance ttravelledby the cradle. in double purchase you will also need double the weight in the cradle that is on the bar (and double that from single purchase of the same weight item)

 

both of the systems work on the same pprincipleand are often combined when there is an obstruction of part of the counterweight cage but some can travel full distance.

 

if given the choice when possible most ppeoplewill go for single purchase as it requires much less effort when doing maintainance and when loading or un loading the cradle.

 

hope this helps and doesn't confuse more, I will try to dig up some of the working diagrams I have of both types of system as they may well help explain better.

 

Ben

[theroo]

hey, yes that helps a lot.

 

it would be really great to see some working diagrams, as im trying to calculate forces in my head and am wondering why double the weight is needed for a double purchase.

 

thanks!

[Tom]

As the load doing the work only travelles half the distance as that travelled by the load you are lifting, it needs to be double the weight.

 

Are you actually dealling with a counterwight system? The other use for a double purchase system might be in a hemp set up where you would rig it the other way around so as only half the force is required by the flyman to lift the load, but for every meter of line hauled the load would only lift 1/2 a meter.

 

THIS site has lovely gifs showing the use of variouse types of rope and pulley set ups. It's aimed at sailing rigs rather than modern theatre but the principals are the same - in fact, that's where we got it all from in the first place.

[Ellis]

From an operational pint of view, if the hauling line is connected to the cradle of a double purchase system (normal arrangement) the amount of force required to overhaul is halved, meaning that it is harder to feel the change in resistance when a light item such as a cloth is fully in.

 

I have only operated counterweights on the ABTT Course - and that was double purchase (feeling the pint when the 3/4 blacks were in was actually quite difficult)

 

Ellis

[robloxley]

Also with double purchase systems there's a lot more intertia to overcome to get anything moving or to stop - doing tab bounces when you have to accelerate the tabs, tab-track and double that in counterweights is fun!

[paulears]

last post about inertia is probably the most important here - damn hard reversing direction in a hurry heavily loaded - worse with heavy drapes. I've always found getting the balance right easier on single purchase systems - I guess the extra rope and pulleys = more friction.

safety wise, I've always hated systems that have operation from anywhere other than stage level. My pet hate is when you do those dance shows with zillions of kids and parents who get in the way.

[Martin]

Indeed, any extra pulleys in the system will add significant friction. Diverted single purchase systems can often be just as bad as double purchase systems. But sometimes it can be a good option to divert a few line-sets to make way for a fire-exit etc., rather than using double purchase. Which personally I hate!

 

One other thing to note is that when you pull down on the hauling line, you fly the bar in. Which is of course the opposite to hemp flying. Just don't forget which system you are using! :o

 

Also the hauling lines on double purchase systems move at the same speed as the bar, even though the cradle travels half the speed/distance. So, operationally, this is the same as a single purchase system.

 

paulears wrote:

safety wise, I've always hated systems that have operation from anywhere other than stage level. My pet hate is when you do those dance shows with zillions of kids and parents who get in the way.

 

In my experience I have found very few venues where the flys are operated from stage level. Generally there just isn't the space in the wings. Also having a fly-floor, keeps everything out of the way of curious fingers! But yes it does mean that you have to have a 'spotter' at stage level, for some amateur productions.