PROBLEM IN HANGER DESIGN

Could anyone please tell me how to solve the following problem:
"Some designed hangers may be supporting zero weight loads."
"Constant effort spring selected,operating load did not fall within allowed working range."

thanks in advance.

I thinkc no one can tell you why unless they can review your model and find the problem.

For point (1): Rest supports are close by.The program arrives at the spring hot load based on dead wt. distribution of the system at " no temp" condition.So if two rest supports ( one of them or both of them may be springs ) are close by , the dead wt. distribution can result in a zero or near zero load on one of them.Solution: space the supports apart.This can also be solved by using user designed springs at location, but that in my opinion is not generally a good design as the system dead wt. distribution should not be disturbed much.

2) The most general reason for this flag is a variability greater than 25 % ( or whatsoever is the value mentioned in the spring spreadsheet)or if the load could not be met by a spring based on variability and space ( if mentioned) within the working range i.e. the load falls in the "lift off" or "bottomed out" areas of the load range. .Solution: try a variability higher than specified value.I have spoken to many spring vendors and I feel that if they agree , although we should try to control the variability between 5-15% close to strain sensitive equipments, (just my opinion , I cannot provide any data or published work to support this), we can exceed the variability beyond 25 % ( 25 % comes from an MSS-SP recommendation)and solve the problem.But I would not recommend exceeding the variability beyond 35 % and suggest that if the variability exceed this no. , better go for a constant spring.

The other reason can be to use the user defined spring ( a variable spring if constant spring is not desired )

Regards

In the last sentence of my response read " solution" in place of "reason".

The zero weight message is displayed when the initial weight analysis (with a hard Y restraint at the hanger location) shows a upward load at that point. Anindya gives a good reason for that.

Don't just use those results though. If there is a positive load at that location, then there must be a bigger downward load elsewhere (we are in equilibrium). You can see, then, that another spring may be oversized. Fix the model so this message does not appear; don't simply ignore it.

We've added some assistance here. You can now view results of this "restrained weight" case (L1 in jobs with hanger sizing). In Analysis, click on the Load Case Options tab. Change the Output Status for L1 from "Supress" to "Keep". You can then see how these deadweight loads are distributed throughout the system.

Again, fix the problem, don't ignore it.

One other thing that you need to look at is your support distribution. As Dave said, if Caesar shows an upward movement at your spring support, you're not going to get a spring sized. A spring won't push down (as we use them).

So, what you may have is a case where you have so much weight on one side of a support, that you are getting a see-saw effect resulting in a upward movement at your spring support. This would mean that you need another support to take up the weight.

Also if you have the spring next to an equipement nozzle and have not freed the nozzle I could see it acting as a Y support (as per Anindya's comment)...

-Shannon