Closed Loop Pulley System, which scenario is better?

[RubenOrt]

Hi Friends,

We are stuck between two scenarios here on a closed loop shade system we are putting together. First, the way this system works is there is a roller containing a shade panel wrapped around it (just like a window shade) and the shade panel connects to a hem bar. Then a steel cord connects to the hem bar and wraps around a pulley and then wraps back around the end of the shade roller to take up the cord.

In this loop there has to be a spring to keep a constant tension on the shade panel so it remains wave free when it is rolled down. It has about 10 lbs of tension to keep the panel wave free when rolled down. When the shade panel is fully rolled up, the spring has about 18lbs of tension on it. The reason the tension is higher while the shade is rolled up is because as the shade panel wraps around itself on the roller, the diameter gets larger thus stretching the spring to compensate for the bigger diameter.

This system will have the shade rolled up for most of its life keeping a constant 18 lbs. of tension on the spring. Every once and a while the shade will be unrolled and the spring will need to have a constant tension of 10 lbs. on the panel to keep it wave free.

Our question is where to place the spring in the loop? Most companies that make a similar system connect the spring to the hem bar (Exhibit B). We need to attach the spring on the back side of the loop system for better access (Exhibit A) We want to try to get the most life out of the spring for as long as possible while maintaining a good tension for its life.

Is there going to be any differences for one method to the other here?I attached a drawing to better describe. Thank you in advance for your help.

http://www.loutrixrecords.com/spring-tensions.jpg"

 

[FredGarvin]

The only mechanical thing I can see I think is a result of a drawing that is not to scale, is will there be issues with pulley friction and losing some of that preload to the pulleys? In other words, will you only be preloading the pulleys and not the shade itself. Would it be possible to look at using a floating pulley on the bottom that you could apply a constant tension force to. That way the pulley applies the load to the shade?

 

[stewartcs]

Our question is where to place the spring in the loop? Most companies that make a similar system connect the spring to the hem bar (Exhibit B). We need to attach the spring on the back side of the loop system for better access (Exhibit A) We want to try to get the most life out of the spring for as long as possible while maintaining a good tension for its life.

Is there going to be any differences for one method to the other here?

The tension will effectively be the same throughout the cable, so the position of the spring in the cable won't matter.

With a static load the spring will last longer with less applied tension. With a dynamic load the fatigue on the spring is a function of the range of tension applied. Since you state that the spring will spend most of its life in a statically loaded condition, the fatigue on it will depend upon the magnitude of the applied tension.

CS

 

[RubenOrt]

Thank you for your responses. We will be using a 5" long spring with a .50" diameter. The initial tension on the spring is 8 lbs at the first 1" of stretch and then 5 lbs per inch after the first inch. The spring max load is 26 lbs. and we will be keeping 18 lbs. max on it. So it should be fine and have minimum fatiguing.

Ruben