Author Options:

Block and Tackle Help - Adjusting Lift Answered


I am working on a project that will lift up a flap, that is mounted to the bottom of a screen door, when the door opens. I am using a block and tackle to do the lifting when the door swing open. Everything works as expected but I would like the flap to travel more, move up/down more but I am not sure how to accomplish that.

I the attached image, the red X's indicates a non-moveable part. The 2 black circles are pulleys. The yellow line is the rope. The green up/down arrow is the part that I would like to raise up higher.

Is there a way to do that without getting too complicated?




1 year ago

Lift up means striaght up like a shute or do you mean the classic type as on all the pet doors?
From your image I assume you want straight up:
You can use a gearbox driven by the door hinges and a worm drive on the flap move it up.
Ok, just joking, would be total overkill ;)
You problem is leverage and travel distance covered by the screen door.
Not really good at drawing so I take your mounting crosses as reference here and number them from left to right.
One is on the door frome, two on the corner of the screen door, three for the flap end after the pulley.
If you move point one away from the hinge area of the door and toward the lock side of the dor the travel distance will increase.
I prefer to use 80LBS braided fishing lines for these things and a cheap ring repair kit for fishing rods.
This way there is no need for pulles and everything is much smaller.
If you can find a cheap fishing reel you can even salvage the roller where the line goes through to make a very small pulley in case there is a lot of weight to move.
Since you secure the line in the door frame "ceiling" you won't have a problem with the screen door damaging the line.
Simply hold you line by hand on the door frame (point one) until you get the amount of movement you need, then fix the line in place.


Reply 1 year ago

Thank you Downunder35m, that did the trick. What was surprising to me was that I didn't have to move the point that attached to the door frame very far from the hinge, only like an inch or two. Just a little move forward made a huge difference.

Thank you!


Reply 1 year ago

Glad it worked out so easy for you :)
Sometimes it doesn not need much and it like with the forest...
Often you can't see the forest because all the trees are blocking the view ;)


Reply 1 year ago

Thank you for the reply. I think I understand what you are saying. I will give it a try this weekend and report back. Thanks again!!!