Originally Posted by NarayananNalinakshan

We have two Stacker Reclaimers with stacking capacity 5500TPH and

loading capacity 3500 TPH for Coking coal/Lime stone.

Boom length is 41 mtrs.Height of stack 10 mtrs.The slew is with 18KW,

VVF drive through planetary gear box.

The coupling between motor and gear box is through a SLIP CLUTCH.

This Slip clutch frequently fails.The HT bolts inside clutch breaks and

the coupling become free.Can any one please suggest the probable reasons.

Narayanan Nalinakshan.

Please explain further with photos if possible.

My first thought would be to examine slip clutch to see if the clutch

or its wear surfaces are in need of replacement.

If the clutch components are worn beyond the factory spec., you have your answer

as a slip clutch is a consumable wear part.

My second thought is regarding the bolts for the stacker reclaimer,

wherein determining if they are the proper grade of bolt and then

examine the torqueing procedure and assuring that the bolts are properly

torqued with a dry torque or lubricated torque value.

This also goes along with assuring the bolt heads are wired to assure they

do not loosen up and fail.

The last issue is operator control as the operator may be forcing the reclaimer

beyond what the slipping clutch is able to compensate for and then breaks.

It is always better to have an overriding speed control that simply allows a function

of a machine to operate at (one speed and one speed only) to remove the human factor of

hurry up and go.

A slip clutch will put up with a lot of abuse, but they do wear out especially if a machine is

pushed beyond its normal desired operating speed. ■

Originally Posted by NarayananNalinakshan

.......

This Slip clutch frequently fails.The HT bolts inside clutch breaks and the coupling become free.Can any one please suggest the probable reasons.

Narayanan Nalinakshan.

The bolts are not up to the duty! 'Why not' cannot be determined from the meagre information provided in the best interests of mysticism. ■

Well, I am very late to the party! Nevertheless, perhaps my comments may help if a future reader of this post is faced with a similar problem...

The earlier comment "The bolts are not up to the duty!" is not only simplistic and naiive, it's also one which unfortunately always comes up as one of the first reasons for failure. Most otherwise "enlightened" people simply do not understand the bolting process. Another comment in this thread ("examine the torqueing procedure and assuring that the bolts are properly torqued with a dry torque or lubricated torque value") supports this seemingly arrogant claim. Alas, this common technical ignorance also extends to the OEMs themsleves when they propogate the false engineering myth of "torquing".

I will take a guess and state that the majority of the bolts likely failed in fatigue. Am I correct, Mr Narayanan Nalinakshan?

When preloads (ie; bolt tightness) are less than the dynamic loads within the system (ie the forces conspiring to separate the joined components), the fasteners begin to cyclically 'flex'. If this continues, at some point the affected bolts will fail in fatigue. When this occurs, load is transferred to the adjacent bolts. Even if these bolts were lucky enough to have the correct preload, they will also begin to flex (since local forces have now become greater) and eventually fail in fatigue. Then, the "domino effect" continues with repeated fatigue failure until there is so much load remaining on the last bolts that they don't fail in fatigue but rather in tension. At that point, the game is over: Forced shutdown.

When faced with this explanation, many people often say that there is no way that their bolts were too loose since they had all been properly tightened with a torque wrench (often even a calibrated one!). This is nonesense; In all likelihood, the bolts were not tightened properly. "Properly" means not too tight or nor too loose but rather, "just right". Assuming that the design engineer had correctly determined how tight "just right" needs to be, the problem is that you're likely not getting close to "just right" even though your mechanics followed the "correct" torquing procedure.

Consider the following "experiment": Lock a bolt's head into a vice. Thread a nut onto the end of the bolt. Spot-weld the nut to the bolt. Set your torque wrench to any value. Then, apply the wrench to the nut and apply force. If it's a dial-indicating torque wrench you'll clearly see the indicator clocking commensurate with the amount of force that's being applied. Stop when you've reached your target torque. Now, sit back for a moment and think about what you just did... You applied the right torque with a calibrated torque wrench. Based on conventional misguided "wisdom", if this bolt/nut combination was actually holding two halves of a flange together, the component would be tight, right? Wrong, wrong, WRONG!

One has to remember that torque is only a measurement of resistance felt as a fastener is tightened. As a previous comment implied, lubrication is a factor (but only one of many). However, it's not as simple as suggested: With not enough lube, the "proper torque" may result in a fastener that's too loose. Too much bolt goop and the fastener becomes too tight. An obvious problem is just how much goo does one apply to the fastener? Where is it applied? What if the goo is contaminated? What if the threads are damaged? What if the thread tolerances are too loose or too tight? What if the joint is sprung? What if the components are rusty? And on and on...

It should become crystal clear to those who have a modicum of engineering sense that the relationship of torque to actual bolt load is a very dubious one at best. As such, it's quite common to find actual bolt stresses on similar fasteners in the same flange to be vastly different than their neighbours - even though they had all been torqued to the same value.

The torquing process forces one to assume that a certain hydraulic operating pressure or a certain beam deflection will result in a specific pre-load. Readers should now agree that this is Ludicrous! Such unrealistic assumptions place significant risk on the future reliability and safety of a plant. The alternative to assuming or guessing is knowing. Thus, the risk of failure is removed. What one must do is measure the effect of the process after tightening. Then, the inevitable inconsistencies can be dealt with by modifying the bolt tightening force accordingly. This must be done for each bolt.

We do this all of the time on critical fasteners where the bolt stress (again: bolt "tightness") must be known and must be very accurate. The process is as follows: Each fastener is assigned a unique Identification Number. These IDs are recorded in the instrument's memory. We then measure the length of each bolt before it is tightened in order to obtain a reference datum. This can be done hours or weeks before actual component assembly. After the fasteners are installed and initially tightened, they are measured again. Wherever we find that the elongation is not sufficient, additional tightening force is applied (either more "torque" or more tensioner pressure or, additional turns of the jacking screws) until the correct elongation has been achieved. Since the data remain in the device (or on any computer onto which the data have been backed-up), this method makes it very easy to check the stress of any fastener at any time in the future just by referring to the reference length and re-measuring again. Obviously this can't be done on coupling bolts while the unit is running but, on static applications such as head cover bolting the procedure can be done at any time. A further benefit is that in the hands of experienced technicians, the process not only results in accurate bolt loads but, it can often identify material defects such as cracks and voids. Thus, during the course of measuring bolt stress, potentially faulty bolts are also identified and can thus be replaced prior to failure.

Yes, the integrity of some joints can be based on assumptions, hopes, wishes and prayers. However, others actually require one to know how tight the bolts are. Unless, of course, one is comfortable with the risk of forced shutdown. ■

Originally Posted by jmalbrecht

Well, I am very late to the party! Nevertheless, perhaps my comments may help if a future reader of this post is faced with a similar problem...

The earlier comment "The bolts are not up to the duty!" is not only simplistic and naiive, it's also one which unfortunately always comes up as one of the first reasons for failure. Most otherwise "enlightened" people simply do not understand the bolting process. Another comment in this thread ("examine the torqueing procedure and assuring that the bolts are properly torqued with a dry torque or lubricated torque value") supports this seemingly arrogant claim. Alas, this common technical ignorance also extends to the OEMs themsleves when they propogate the false engineering myth of "torquing".

I will take a guess and state that the majority of the bolts likely failed in fatigue. Am I correct, Mr Narayanan Nalinakshan?

When preloads (ie; bolt tightness) are less than the dynamic loads within the system (ie the forces conspiring to separate the joined components), the fasteners begin to cyclically 'flex'. If this continues, at some point the affected bolts will fail in fatigue. When this occurs, load is transferred to the adjacent bolts. Even if these bolts were lucky enough to have the correct preload, they will also begin to flex (since local forces have now become greater) and eventually fail in fatigue. Then, the "domino effect" continues with repeated fatigue failure until there is so much load remaining on the last bolts that they don't fail in fatigue but rather in tension. At that point, the game is over: Forced shutdown.

When faced with this explanation, many people often say that there is no way that their bolts were too loose since they had all been properly tightened with a torque wrench (often even a calibrated one!). This is nonesense; In all likelihood, the bolts were not tightened properly. "Properly" means not too tight or nor too loose but rather, "just right". Assuming that the design engineer had correctly determined how tight "just right" needs to be, the problem is that you're likely not getting close to "just right" even though your mechanics followed the "correct" torquing procedure.

Consider the following "experiment": Lock a bolt's head into a vice. Thread a nut onto the end of the bolt. Spot-weld the nut to the bolt. Set your torque wrench to any value. Then, apply the wrench to the nut and apply force. If it's a dial-indicating torque wrench you'll clearly see the indicator clocking commensurate with the amount of force that's being applied. Stop when you've reached your target torque. Now, sit back for a moment and think about what you just did... You applied the right torque with a calibrated torque wrench. Based on conventional misguided "wisdom", if this bolt/nut combination was actually holding two halves of a flange together, the component would be tight, right? Wrong, wrong, WRONG!

One has to remember that torque is only a measurement of resistance felt as a fastener is tightened. As a previous comment implied, lubrication is a factor (but only one of many). However, it's not as simple as suggested: With not enough lube, the "proper torque" may result in a fastener that's too loose. Too much bolt goop and the fastener becomes too tight. An obvious problem is just how much goo does one apply to the fastener? Where is it applied? What if the goo is contaminated? What if the threads are damaged? What if the thread tolerances are too loose or too tight? What if the joint is sprung? What if the components are rusty? And on and on...

It should become crystal clear to those who have a modicum of engineering sense that the relationship of torque to actual bolt load is a very dubious one at best. As such, it's quite common to find actual bolt stresses on similar fasteners in the same flange to be vastly different than their neighbours - even though they had all been torqued to the same value.

The torquing process forces one to assume that a certain hydraulic operating pressure or a certain beam deflection will result in a specific pre-load. Readers should now agree that this is Ludicrous! Such unrealistic assumptions place significant risk on the future reliability and safety of a plant. The alternative to assuming or guessing is knowing. Thus, the risk of failure is removed. What one must do is measure the effect of the process after tightening. Then, the inevitable inconsistencies can be dealt with by modifying the bolt tightening force accordingly. This must be done for each bolt.

We do this all of the time on critical fasteners where the bolt stress (again: bolt "tightness") must be known and must be very accurate. The process is as follows: Each fastener is assigned a unique Identification Number. These IDs are recorded in the instrument's memory. We then measure the length of each bolt before it is tightened in order to obtain a reference datum. This can be done hours or weeks before actual component assembly. After the fasteners are installed and initially tightened, they are measured again. Wherever we find that the elongation is not sufficient, additional tightening force is applied (either more "torque" or more tensioner pressure or, additional turns of the jacking screws) until the correct elongation has been achieved. Since the data remain in the device (or on any computer onto which the data have been backed-up), this method makes it very easy to check the stress of any fastener at any time in the future just by referring to the reference length and re-measuring again. Obviously this can't be done on coupling bolts while the unit is running but, on static applications such as head cover bolting the procedure can be done at any time. A further benefit is that in the hands of experienced technicians, the process not only results in accurate bolt loads but, it can often identify material defects such as cracks and voids. Thus, during the course of measuring bolt stress, potentially faulty bolts are also identified and can thus be replaced prior to failure.

Yes, the integrity of some joints can be based on assumptions, hopes, wishes and prayers. However, others actually require one to know how tight the bolts are. Unless, of course, one is comfortable with the risk of forced shutdown.

OK Smarty. Now explain to the rest of us simplistic naive morons just how the application of some fancy bolting method stops the clutch failures. As a modicum of European Engineer(ing) sense the earlier reply was a simple statement of fact. Maybe a larger coupling with more bolts and more casing is required. Until the punter tells us more, rare enough in these forums, the strength of the bolted assembly remains the sole indicator of the failure mode. Perhaps the thread starter would care to ascertain that after applying the criteria suggested above the failures have indeed stopped. Until then we should all go back to sleep on this one. ■

Dear jmalbrecht, louispanjang,

I second the statements of Mr. jmalbrecht.

An applied torque on a bolt(nut) does not mean that you have pre-tensioned the bolt to a required stress.

The unknown factor between the two is the friction.

In addition, the suggestion of Mr. jmalbrecht

, the majority of worldwide engineers is ignorant, is not true and a bit arrogant.

Although, I am not feeling offended.

The inability in practice to “torque” equipment in the field properly is normally compensated by overdesign.

Where accurate tensioning (or pre stressing) is required, Mr. jmalbrecht’s company can help.

Together with louispanjang, I would like that thread starters come back with valuable feedback from their experiences after they have checked or applied the given advises from the forum members.

Too often, a thread ends in ?????.

Have a nice day

Teus

(Still enjoy this forum) ■