Re: Crowned Tail Pulley
Shane,
If you use a crown faced pulley, the differential diameters will cause an increase in the tension in the centre of the belt and a reduction in tension at the edges. As steel cord belts have a high elastic modulus, these changes in tension will be significant. If you look at the relative path lengths around the pulley and consider the induced strain, you will see why crown pulleys are not normally recommended for use with steel cord belts.
Regards,
David Beckley,
Conveyor Design Consultants of WA
Perth, Western Austrailia. ■
Re: Crowned Tail Pulley
Dear Mr. Shane,
Mr. David Beckley has given the engineering reason for unsuitability of crowned pulley with steel cord belts. I am putting the same thing in a simplistic manner as below.
The textile fabric is comparatively stretchable, so, it can accept differential elongation easily and set properly on the crowned pulley, with lesser value of stress related to strain. However, steel cord belt is less stretchable and it will not easily accept the differential elongation between belt edges and belt center, easily. This will cause large difference in stresses at the belt center and stresses at the belt edges, which is objectionable.
Regards,
Ishwar G Mulani.
Author of Book : Engineering Science and Application Design for Belt Conveyors.
Email : parimul@pn2.vsnl.net.in
Tel.: 0091 (0)20 25882916 ■
Re: Crowned Tail Pulley
As said in the replies the center of the belt will be subject to stronger tensions in Steel cord conveyor.
If the belt may resist , the weakest point is always the splice will not, in the center of the belts if later you Xray the splice you for sure will observe cord end pull and destroy adhesion between Cord and rubber in the center cables of the splice.
Later you will see some protuberance on the surface and any time your splice is gone ■
Crowned Tail Pulley
OK....this is clear, but bucket elevators supplier crown both the drive and tail pulleys so that they track well.
Why is this not detrimental??
Thanks
James ■
Re: Crowned Tail Pulley
Yes, as Mr. R J Morrish says, the crowning is used for steel cord belts in bucket elevators. In this case also, the phenomenon as mentioned earlier holds good. However, this is countered by higher safety factor somewhere around 9 to 10 instead of 8.
The crowning used for steel cord belt in case of bucket elevators, is possible less sharp than conventional values.
Above information is as per published literature of the belt manufacturers.
Regards,
Ishwar G Mulani.
Author of Book : Engineering Science and Application Design for Belt Conveyors.
Email : parimul@pn2.vsnl.net.in
Tel.: 0091 (0)20 25882916 ■
Why Crown Pulleys?
Dear Shane
I believe that the issue should be viewed from the point that since crowning is intended for better belt tracking but that it is bad for the belt, you are still left with the issue that the belt is still running off centre or theat the possibility exsists that it will.
One therefore has to find products that can steer a belt back to centre but the product should be belt friendly.
Take a moment to view the products at www.waloni.com and see what we have to offer. Waloni has made belt tracking there core business and concern and have solved thousands of belt tracking problems since 1992.
Hope we can help you.
Regards
Mark Hupertz ■
Re: Crowned Tail Pulley
I don't believe the added fatigue on the splices and belt is worth the minimal belt training effect of a crowned pulley. This is true for fabric or steel belt and for elevators and conveyors.
If properly designed, manufactured and installed you don't need crowned pulleys. I would not use a crowed pulley on any high belt tension application especially steel cord. ■
Crowned Pulley
Dear Mr. Shane,
Crowining means you are providing different diameter at centre & at edge.Due to this, belt speed will be different & lead to tension difference & hence shotter belt life.
Regards.
A.Banerjee ■
Strange Concerns
Gentlemen,
I am quite surprised about these arguments. A standard tail pulley, 1600 mm wide for instance, has a difference in diameter between centre and side of about 2,4 mm. The additional tension we are talking about is just enough to centre the belt properly and reliable.
The additional belt tensions in the centre are much less than those caused by the troughing of the belt in front of that pulley even if the standard length of the troughing area is kept as per the belt suppliers specifications (which is at least on mobile machines quite often not possible).
So I see in the crowned pulley one of the cheapest and most reliable solutions to keep the belt on track.
Holger ■
Re: Crowned Tail Pulley
Dr Lieberwirth,
What would you predict the induced strain would be if the diameter of the tail pulley was 600mm at the centre and 596mm at the edge and the belt strength was ST 1000 kN/m? If the elastic modulus was 65000 kN/m what tension variation whould you predict from the belt edge to belt centre?
I look forward to your reply.
David Beckley.
Conveyor Design Consultants of WA
Perth, Western Australia. ■
Crowned Tail Pulley
Dear Mr. Shane,
It is simply obsolite design.
Regards.
A.Banerjee ■
Re: Crowned Tail Pulley
Hello Shane..et al
There are three types of crowning:
-Full crowning at 1:100 from edge to centre
-Curve crowning (old method) where the end of the shell is dome shaped
-Edge crowning where only the last 10% or so is crowned at 1:100 on each side.
The fact is that edge crowning and full crowning have exactly the same effect. This is because the training forces in the central part of the shell balance out.
It is only the difference of the effective widths on the sloping edges of the pulley that creates the training force.
Edge crowning can be used on any type of belt, as 80% of the belt is on a flat pulley.
Remember, if you:
-Never use training idlers
-Align the pulleys
-Have flat pulleys or only edge crowned ones
-Align the idlers
-Keep build up to a minimum
-Splice the belt straight
-Load centrally
Then all will be well
But
Crowning pulleys cannot overcome de-training resulting from the above
Regards
LSL Tekpro ■
Theoretic Calculations
Dear Mr. Beckley,
it seems to me that your question is targeting into the wrong direction. The maximum tension which might occur at the return pulley is determined by the whole design of the conveyor system the forces acting in the system and the way the drive and the tensioning device are operating. So the tension difference between the centre and the edge of a belt at a crowned pulley does not play a significant role in the durability of a properly done belt splice. It is just enough to keep the belt on track.
So, as Mr. Spriggs explains there may be different alternatives for the growning and it is certainly a very old principle (at least dating back to the flat belts of the steam engine period!) to centre belts. But it is still working extremely simple and effective. So it is by far not outdated or obsolete. At least the long distance horizontally curved belt conveyors we supplied some 20 years ago to Halla cement in Corea (C-C distance >4 km and transport in upper and lower strand) are still working perfectly with these crowned pulleys as well as hundreds of other conveyors and belt bucket elevators which have been supplied by Beumer.
Holger Lieberwirth ■
Re: Crowned Tail Pulley
Graham,
Why dont you recommend the use of training idlers
Cheers
Shane ■
Re: Crowned Tail Pulley
Hello Shane..
I reckon that training idlers are a great idea,.. as long as you never actually try and use them.
They have to be put in the right way round, have the central pivot designed and installed in the exact right place in relation to the rollers, and they must be of the in-line roll configuration.
Even after ensuring all the above I have never had any luck with them and have abandoned them in favour of properly aligning everything. (This works well)
In fact I have on countless occasions seen most training idlers wired into a fixed position or siezed into a fixed position.. so why spend the money on them.
If you have a bad appendix you remove the problem.
You do not just take pain killers.
Same with belt training.
Regards to you
LSL Tekpro ■
Re: Crowned Tail Pulley
Holger,
I think that the locally generated stresses are important. Looking at the example from my previous posting on this topic, which refered to a pulley diameter of 600 tapering to 596mm. In simplistic terms the strain that will be induced from the tapered pulley shell will be proportional to the variation in pulley diameter, which in the example is 4/600. In conveyor engineering I believe that the elastic modulus expressed in kN/m will be equal to the tension variation expressed in kN/m divided by the strain. For the example, the tension variation will therefore be 4x65000/600 = 433kN/m. Depending on the shape of the crowned pulley, 50% of this tension variation will increase the belt tension at the centre and 50% will reduce the belt tension at the edges. This is a significant tension variation for an ST 1000 belt that has a rated working tension of only 150 kN/m! and that will probably only have a working tension at the tail pulley in the order of 50 kN/m I realise that the strain will actually occur over a longer length either side of the pulley and this will reduce the magnitude of the tension variation and that the shape of the tail transition will also counteract on the tension variation caused by crowning the pulley. However I am a firm believer in looking at total stress levels and not just the working belt tension at all major parts of the conveyor, the new version of DIN 22 101 goes part way to support this method of analysis of peak belt tensions. Many people also use a 1/100 taper, which will result in even larger tension variations than the above example.
In conclusion, I believe the above example demonstrates that significant belt tension variations will occur as a result of using a crown faced pulley with a steel cord belt and as one should endeavour to avoid overstressing the centre of the belt and also to avoid zero tension at the belt edges, I support the belting manufacturers in generally opposing this practice.
I wonder if any of my learned collegues will like to comment on these tension variations and the methods that they use to analyse the magnitude of the variations .
David Beckley
Conveyor Design Consultants of WA
Perth, Western Australia. ■
Re: Crowned Tail Pulley
Shane,
I generally agree with Grahams comments. On Hamersley Iron's two 10km overland conveyors there are only a couple of training idlers prior to the tail pulley and also in the take up area. For the rest of these conveyors there are no training devices other that 'V' return idlers. In fact I went to great length by using jig assembly techniques to ensure that all idlers were set square to the conveyor centreline and in fact there is no provision for the fitters to adjust the position of the trough idlers so they are all still parallel. These conveyors have now operated sucessfuly for 16 years.
You will find that return training idlers will work reasonably well but not trough trainers as the gravitational pull on the mass of the conveyed material will have a greater affect.
Get everthing square and level, center the belt prior to the loading area, centalise the load and most importantly avoid any lateral bias in the load at the loading point and all should be well.
Dave Beckley. ■
Re: Crowned Tail Pulley
Dave,
Completing your thought with the additional aberation of lagging damage and pulley shell stress levels that may exceed the nominal belt tension. The pulley shell rating will certainly need to be increased. ■
Re: Crowned Tail Pulley
Hi there Dave..
Re:
"In conclusion, I believe the above example demonstrates that significant belt tension variations will occur as a result of using a crown faced pulley with a steel cord belt and as one should endeavour to avoid overstressing the centre of the belt and also to avoid zero tension at the belt edges, I support the belting manufacturers in generally opposing this practice."
As I mentioned earlier, this does not apply to any relevant extent if you only crown the edges of the pulley, and the training effect will be exactly the same as if you crown the entire face width, as the bit in the middle cancels itself out.
Regards
LSL Tekpro ■
Re: Crowned Tail Pulley
Graham,
I am not sure what you mean by the bit in the middle cancels its self out.
In a situation where there was very little space for a tail transition and centre buckling was occuring that caused splice separation, I suggested to a client that they try a crowned pulley where the centre section is flat for a length equal to the length of the centre idler roll and with the edges crowned to a diameter that will induce an equal and opposite tension variation to that being caused by the transition. I am not sure what all these changing tension variations and the associated shear stresses in the separation rubber do to the splice but it did apparently solve the edge buckling and splice failure problem.
I stress that this is the only circumstance that I would suggest the use of a crowned pulley with steel cord belting.
Dave Beckley ■
Re: Crowned Tail Pulley
David..
Suppose your belt is 1000mm wide.
Suppose also that the belt has wandered to the left side of the pulley by 50mm.
You will then have 550mm of belt width on the left side of the pulley and 450mm of belt width on the right side of the pulley.
There is therefore 900mm of belt about the centre line of the pulley which is symmetrical (2 x 450mm each side) and all training forces from the left towards the centre line balance out with those from the right towards the centre line for this "bit in the middle cancels itself out" (where crowning is a waste of time).
However, you do have 100mm of belt on the left side (1000mm - 900mm), which is trying to climb towards the centre of the pulley that has nothing to counter it on the right side. This 100mm will always be on the crowned portion even if only edge crowned.
Therefore a simple bit of edge crowning is the best and does not have any bad effect on the centre of the belt.
Interesting.. but quite true.
LSL Tekpro ■
Re: Crowned Tail Pulley
Graham,
I think we are talking about two different things here. You are talking about balance betwen the correcting forces from the left and right hand sides, while I am looking at belt tension variations caused by the difference in diameter. For a 1000mm wide belt and a pulley width of say 1200mm, how far in from the edge do you crown? If it is 300mm at 1/100, it is still a difference in diamater of 6mm, which will cetainly cause large tension variations in the belt. With this type of crowned pulley the calculation of the high and low tensions is more complex but is still solvable.
Another point, that I don't think anyone else has commented on and I apologise if they have, is that crowning must cause significant pulley lagging wear and to a lesser extent belt cover wear.
Dave Beckley. ■
Re: Crowned Tail Pulley
David..
As I mentioned earlier, 10% edge crown either side will do.
10% of 1200mm =120mm.
At 1:100 this is 2.4mm difference on the diameters.
Compare this if you will to full crowning at 1:100 which gives you 12mm difference which would be terrible for the belt, and loose you 6mm of shell thickness too.
When you train the belt it runs in the middle of the pulley.
Remember that the middle bit which is flat is 1200mm less 2 x 120mm i.e. = 960mm. In other words the widths of the edges of the belt on the sloping edges are both 1000mm less 960mm divided by 2.
This gives a meer 20mm each side for which the difference in diameters is only .4mm !
If you edge crown only 100mm each side then this value =0
This is miles better than the equivalent value for full crowning which would be 10mm as a direct comparison.
I still normally do not bother with crowning, and as you know, I prefer to align things nicely instead. This is also in line with my personal preference for ceramic lagging epoxied onto the shell, where achieving an edge crown is not at all easy in practice.
It is difficult enough to do a nice tiling job on a flat pulley!
LSL Tekpro ■
Crowned Tail Pulley
I realise crowning a tail pulley on a steel cord belt is not recommended practice.
Can someone please explain to me why, and the possible consequences.
Regards
Shane ■