I recently had a shaft break down and I have problem to understand the reason for it. This cardan shaft below drives an auger. At the drive end there is a gear box with a 500kW driving it. The maximum torque at the drive end is 23kNm. This is the locked rotor torque of the motor (5071Nm) multiplied by the gear ratio (4.5623). The required torque for break down of the shaft is 52kNm. The auger was stopped and the motor kept pushing until the shaft broke. There is no sign of fatigue or previous mechanical damage on the shaft. There is no damage on the gear box either. The crosses at both ends have broken off. To me it looks like the shaft broke due to over load but the numbers don't add up. The machine has been running for 1200h. Does anyone have an idea?
Where is the system supposed to slip or shear? If the load is forced to a stop it is going to give somewhere. What stopped the machine? Was it a sudden stop, like a piece of iron in a jaw crusher, or did something just plug up? Has it stopped before without damage? What is the puzzle, is the shaft rated for much more than the rated power being put to it?
It looks like the cross failed, the bearing pin area sheared off ?
This one is protected by the motor braker/ soft start equipment. There is also a speed guard that shuts off when the speed dips below 300rpm. This function is disabled during start up to give the screw time to reach its speed. It probably happened at start up. I suspect it is a build up plug up since there is no damage to the screw. The motor kept pushing for 13sec until the soft start shut it down/ shaft broke. Yes the shaft should be able to take twice the torque the motor can produce.
Exactly the pins in the crosses sheared off.
these come to mind as possible reasons: 1. Bad forging with some internal inclusion. 2. Bad machining at pin radius leaving marks. 3. Bad heat treatment caused stress's. 4. Bearing failed & cut the pin dia. making stress riser.
Not that uncommon-The joint is the weakest link, be glad it is a short yoke or it could have split that.
Shock load is exponentially higher than running or even start up. I would speculate that during its "ramp up" or soft start as you described it, the implement was stalled, somehow, with force piling up and no way to unload with more rotational force being applied, it failed the weakest link.
Thanks for all your feedback. I would also suspect the Yoke to be the weakest point. Regarding shock load. Since it broke after the motor pushed for 13 seconds I would not consider it to be a shock load. My assumption is that once the rotor stop torque is passed the motor would theoretically be able rotate backwards.
See if there is a "Made in China" embossed on that ujoint.
Any chance you can upload a pic of your implement and drive. I'm picturing a PTO and a right angle gearbox. Perhaps the implement was positioned in less than ideal angle, causing a mechanical bind within the yokes. Struggling to visualize what you're up against
You said the drive cuts out below 300 rpm. What speed does it normally run at?
Her is a sketch of the drive installation. The connection points of the cardan are fixed and there is no radial movement of the auger. The angle of cardan is less than 5deg. The Auger runs at appr. 400rpm.
Shock load could have tweeked it and this time it couldn't hold, you typically see broken rust, the far right pic kinda shows it.
I am still wondering what the mystery is. You plugged up and stalled the load, and then stuffed 670 horsepower into that shaft for 13 seconds. What was supposed to happen? It is going to fail somewhere, a driveshaft is generally going to fail at either the u joints or the slip yoke. I guess I wouldn't be the slightest bit surprised to find that the machine smoked the u joints when it plugged up and ran for 13 seconds before anybody caught it. Personally, I would be pleased that the weak point is in a easily replaceable point like the driveshaft. I was once involved with a situation where we bent an 8" eccentric shaft on a jaw crusher when a cast iron pillow block got into the rock. That was expensive.
Shock load does occur but nor so often. In normal operation the power to drive the auger is quite low. Appr. 50%.
Yes of course, there is quite a lot of power on the shaft... It is the weakest point but it should be able to take twice the maximum torque that can be produced. The normal power output is maybe 250-300kW. It is designed to take the chock loads. In the future we will install a fluid coupling to protect the shaft.
Why do you believe that the shaft should withstand twice the rated output of the drive motor? What is the rated load for the shaft? The shaft should withstand 1300hp? I don't understand what is meant by "the machine is designed to take chock loads", as in, it is allowed to plug up. Somebody built a machine with an application that calls for 300hp, and rather than putting in a clutch, belts or a fluid drive they just put in a shaft rated for four times the power imput?
I always like the driveline that is twisted/spun to a point like a pencil. A drive shaft is meant to transfer power not be loaded at constant static level.
I guess, I'd rather see a blown drive shaft then a wadded up gearbox or smoked motor.
Nobody has mentioned the nice failure analysis picture collage. Very impressive. A+.
Tank mixer?
Not me-I want to see a hole in the bell and smoke billowing.
“Tank mixer?“. Sure looks like something a blender would use in a plant.
I remember reading that the smart guys who hopped up their semi's deliberately didn't put the absolute heaviest drive shafts in. Much better to have the driveshaft the weak link than the transmission or diff.
I missed the auger part before, that says more screw conveyor to me. I know we've pulled a lot of crap out of screw conveyors before that tore up drives, also operators trying to stuff the wrong product through one.
Is this the first failure, or does it happen all the time? As it has been pointed out, there is rust showing on one of the breaks, and perhaps it was a faulty joint. Stick a new one in and if it fails again....... Might be over thinking it. I'm sure we have all had issues with sub par equipment.
I'm not sold on that joint being fractured and rusting that deep in the cross. I believe that was transfer, either at the time of joint failure or during subsequent handling. I will say, I don't think I've ever been presented with such a detailed layout of failed parts. Usually i get everything in a bucket or crate of filth with a " I don't know what happened? "
Typically it's been blamed on someone else already