Slip Tension...
With T2 calculated from the requirements to maintain 1% sag one must check to see at breakdown, whether the belt will slip of continue to gain tension.
It would unjustified to design a conveyor structure for extreme loadings if when combined with the take-up tension and drive factors the available traction tension is less then breakdown tension.
One must check the slip ratio of the drum, ie there are instants where conveyor systems slip at 200% of full motor torque, hence if you simple calculate the tension which occurs at breakdown, the belt could slip before this breakdown tension. This would result in the structure being over designed and costing more then it should.
Anyway, your comments are just and there is more then one way to look at all problems. I was previously an aircraft design engineer, I have been only involved with conveyors for about 8-months, this forum is extremely informative in matters like these. ■
Re: Conveyor Calculations
When the belt slips on a drive pulley, you enter a more dangerous regime of operation. I have witnessed vibration from slip-stick action of the drive that produce equivalent forces that appear to be mulitples of the motor nameplate and beyond the breakdown torque magnitude.
The vibration and slip became sympathetic with the building structure and began to cause large, visually observable deflections of the building. THe vibration was audible as well. THe drive was on the ground two hundred feet from the 60 foot elevated head pulley. No operator would enter the building when the conveyor started.
The head pulley had failed when we were asked to evaluate the operation. The wound rotor drive motor starting sequence was modified with more rotor resistance and staggered timing. The problem disappeared with the changes.
Side note: this was a PWH Rhinebrown three motor drive design with the first step at 160% of nameplate torque. ■
Re: Conveyor Calculations
Another note on my comment:
THe takeup was a fixed winch type after the drive. We left it without logic and installed a tail gravity takeup.
The conveyor also had a horizontal curve that would not behave. Once the changes were made, the horizontal curve belt action became acceptable. ■
Conveyor Calculations
Mark J.A,
It appears that you have gained much understanding though your summary indicates remaining confusion.
1.) T2 must have been determined for tension to limit sag or some other criteria, clearly not for the available wrap.
2.) Once Te and T2 are known the wrap factor is no longer used to calculate T1, only to know that it is more than capable of developing Te for what the motor can impose.
3.) Mechanical and structural design then follow from these numbers, with appropriate (well published) safety factors.
4.) I generally agree with your concluding statement but only when a conveyor is responsibly designed to conservative criteria, particularly what I would call rigging criteria (design stress fa=Fut/5) at the variable and moving components, at the terminal ends. Nevertheless it is very valuable to know the transient, dynamic loads and the corresponding stresses to vindicate your statement.
Joseph A. Dos Santos ■