There is no difference in rope tension except for the rolling resistances of the sheaves, say T_S1etc.

So, T1+T2=T3=T4 if you prefer to ignore the T_Sx's like most people do anyway. I have never accounted for sheave resistance: yet. Not even in the design of Dover Ro-Ro linkspan where each winder carried 8 falls of rope and the SWL was 760 t. ■

Originally Posted by johngateley

There is no difference in rope tension except for the rolling resistances of the sheaves, say T_S1etc.

So, T1+T2=T3=T4 if you prefer to ignore the T_Sx's like most people do anyway. I have never accounted for sheave resistance: yet. Not even in the design of Dover Ro-Ro linkspan where each winder carried 8 falls of rope and the SWL was 760 t.

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John:

CDI has measured up to 44% tension hysteresis on take-up rope and sheave systems with proper diameter sheaves for give rope when using bushings for sheave bearings. We have measured > 100% on poorly ratio of sheave to rope diameters (12:1 was fatal), which led to rope fatigue failure after about 2 years of service. Best to keep (D/d) ratio of 25:1. Nothing below 18:1 without high hysteresis and potential failure of internal wire strands. Witnessed failure of 15:1 after about 5 years of service.

There are two types of hysteresis: 1) sheave bearings; 2) wire internal friction due to bending and twisting.

Read sheave supplier recommendations % hysteresis by bearing type.

Should only use high flex rope such as 6x37S style. Best to reset rope position on fixed take-up from time to time to minimize local wire fretting, especially with high cyclic stress range due to significant dynamics tension range. If travel has local oscillation in small displacement range, wire fatigue can be an issue. ■

Hello,

The conveyor design establishes the required tension T3 and T4 at tail end. Tension T3 is practically equal to T4, the difference being only the tail pulley turning and belt wrap resistance.

Therefore, the take-up system has to pull tail pulley carriage by force T3 + T4 = 2T3.

So wire rope attached to tail pulley carriage, will have tensile force 2T3. This tensile force will be present everywhere in the rope, right upto the counterweight. Decide counterweight force (mass) according to rope arrangement at counterweight. In your case, the counterweight basic value will be 2 x (2T3). Here counterweight means counterweight including its supporting frame / box.

If you are using twin rope system i.e. 2 numbers of rope pulling the pulley carriage, then above tension value in each rope will become half, but ultimately counterweight quantity will remain same for the arrangement shown by you.

The conveyor belt total length increases / decreases slightly due to variation in load (mtph), steady running, starting and stopping, which causes pulley carriage to shift by small distance. During such situation the take-up force reaching to belt will have some + / - variation due to frictional resistance in wire rope system. The designer can take into consideration such frictional resistance, which will result into increase in counterweight by some percentage so that belt does not slip at drive pulley during worst situation. It is possible that even if one does not take into consideration such finer aspect, then belt may not be slipping due to inherent contingency in the value of friction coefficient being considered between belt and drive pulley and also inherent contingency in conveyor frictional resistance coefficient.

I account for the influence of frictional resistance in sheave rope system.

It is also recommended that plant maintenance people should do regular re-greasing of the sheave bearings, which would improve the life of belt and drive pulley.

Ishwar G. Mulani

Author of Book : Engineering Science And Application Design For Belt Conveyors (new print November, 2012)

Author of Book : Belt Feeder Design And Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916

Email: conveyor.ishwar.mulani@gmail.com

Website: www.conveyor.ishwarmulani.com ■

I note John Gately's mention of friction in flexure of Wire Ropes. Please explain the Tsx's. Am I correct in assuming that they are the rope and sheave frictional components?

I have a figure of 1.5% friction per 'new' sheave, and obviously, lubricated rope. With over 30 years experience in Conveyor Tension Control we are seeing old systems with extreme friction in the rope transport due to rope friction. We have seen sheaves worn through the bottom of the sheave groove and ultimately ending up cutting a 2-inch deep groove in the structure. Does anyone have any figures for rope friction? We are attempting to quantify the total friction in Winch Take-Up Systems.

Our experience is that the friction of Ropes and sheaves grow from insignificance in a new installation to become a monster in time. That time can be months to many years.

Also Take-Up Trolley Wheels without proper bearings start deteriorating within weeks of installation.

Ian Plunkett

Iptron Technology cc, Johannesburg South Africa

Tel: +27 11 534-1285

Email: iptron@mweb.co.za ■

Originally Posted by iptron@mweb.co.za

I note John Gately's mention of friction in flexure of Wire Ropes. Please explain the Tsx's. Am I correct in assuming that they are the rope and sheave frictional components?

I have a figure of 1.5% friction per 'new' sheave, and obviously, lubricated rope. With over 30 years experience in Conveyor Tension Control we are seeing old systems with extreme friction in the rope transport due to rope friction. We have seen sheaves worn through the bottom of the sheave groove and ultimately ending up cutting a 2-inch deep groove in the structure. Does anyone have any figures for rope friction? We are attempting to quantify the total friction in Winch Take-Up Systems.

Our experience is that the friction of Ropes and sheaves grow from insignificance in a new installation to become a monster in time. That time can be months to many years.

Also Take-Up Trolley Wheels without proper bearings start deteriorating within weeks of installation.

Ian Plunkett

Iptron Technology cc, Johannesburg South Africa

Tel: +27 11 534-1285

Email: iptron@mweb.co.za

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Dear Ian,

John G. advised he sees no friction. I advised there can be substantial friction in the rope behavior. The friction comes from contact between wires, the radial pressure and torsional twist exterted on their contact points. Thus, the rope construction can play a large role in the frictional loss as the rope is bent and twisted around the sheave guides.

It can become more pronounces with permanent wire deformation within the rope caused by undersized sheave diameters wrt the rope diameter. ■

Ask any 'Old-Timer' in the Conveyor World about Gravity Towers and you'll get the same story. "When the drive slips during Start-Up you add more weight to the Tower". That hasn't changed. We are often asked to quote a Take-Up Winch System to replace an ailing Gravity Tower.

If a gravity tower has 8 sheaves, this amounts to 8 x 1.5% = 12% friction; this is a combination of Rope Flexing Friction as well as Sheave Bearing Friction. This acts in both directions so the accuracy of tension control is +/- 12%. This is quite acceptable and workable. In the past 30 years, I have seen several hundred Conveyor Tensioning Systems and not of them has any provision for rope lubrication. I have seen plenty of very rusty ropes and several collapsed sheaves.

Extremely common in my experience is Take-Up Trolleys with seized wheels. Now if the average Take Up trolley has a mass of 2000kg, a single seized wheel running on an inverted Angle-Iron Rail,will, on average, add approximately 4kN of friction. I have seen one with 4 seized wheels too! The customer gives the opinion that a little friction does not matter because the trolley will find its own equilibrium. The trouble is that while the tension is changing, and the trolley and Rope Transport are moving, all the frictional components are having a maximum effect on the conveyor tension. No wonder the drive slips!

My small company does not have the equipment to undertake testing of flexing friction in a totally dry or rusty steel ropes. I have contacted Wire Rope Manufacturers and they have no data. Besides they are only interested in new (lubricated) ropes.

Gravity Towers located directly below the (elevated) belt, can have the tensioning mass hanging directly on the Take-Up Pulley. This is undoubtedly the most efficient and least troublesome type of Gravity Tower.

My experience is primarily with Take-Up Winches and every system fails ultimately due to excess friction. In practice a minimum of 2 Rope Sheaves are used, only 1 of which is in the 'Measurement Zone'. However there are Take-Up Trolleys with 4 and even 6 falls of rope. A properly designed Conveyor Tension Control System can actually 'see' and discriminate between Trolley Wheel Friction and Rope Transport Friction.

What is the ideal Tensioning System? ■

Originally Posted by johngateley

There is no difference in rope tension except for the rolling resistances of the sheaves, say T_S1etc.

So, T1+T2=T3=T4 if you prefer to ignore the T_Sx's like most people do anyway. I have never accounted for sheave resistance: yet. Not even in the design of Dover Ro-Ro linkspan where each winder carried 8 falls of rope and the SWL was 760 t.

On a parallel thread I have indicated that a weight of several tonnes moving up and down under the influences of fluctuating tension ought to be treated as a crane load. Both these threads accept the denial that the additive friction created is solely due to neglect. I have designed much heavier cranes and hoists than those little things you talk about. Dover Double Deck Ro Ro is still in continuous operation since it was built in 1985. That's about thirty years. Such an achievement is at the door of the maintenance crew who seem much better able than the guys who stroll around conveyors all day long between tea breaks.

In most hoisting applications rope stretch is either comical or insignificant. Any designer will relieve twisting between the wires by aligning the sheaves at right angles. Why isn't filled rope used if re-lubrication is difficult? The only serious exception to account for rope stretch is on winding gear where it is helpful that the bridle arrives precisely where it is supposed to. American Standards did not consider this small point, in the last century anyway, and that was quite embarrassing for Otis etc in the face of European and Japanese competition.

Simply put: an ideal tension system is anything that is safe, reliable and easy to maintain. None of the ones mentioned seem to meet those criteria so start from there. You're off to a good start because you have at least one grease gun more than your clients. ■

Originally Posted by johngateley

On a parallel thread I have indicated that a weight of several tonnes moving up and down under the influences of fluctuating tension ought to be treated as a crane load. Both these threads accept the denial that the additive friction created is solely due to neglect. I have designed much heavier cranes and hoists than those little things you talk about. Dover Double Deck Ro Ro is still in continuous operation since it was built in 1985. That's about thirty years. Such an achievement is at the door of the maintenance crew who seem much better able than the guys who stroll around conveyors all day long between tea breaks.

In most hoisting applications rope stretch is either comical or insignificant. Any designer will relieve twisting between the wires by aligning the sheaves at right angles. Why isn't filled rope used if re-lubrication is difficult? The only serious exception to account for rope stretch is on winding gear where it is helpful that the bridle arrives precisely where it is supposed to. American Standards did not consider this small point, in the last century anyway, and that was quite embarrassing for Otis etc in the face of European and Japanese competition.

Simply put: an ideal tension system is anything that is safe, reliable and easy to maintain. None of the ones mentioned seem to meet those criteria so start from there. You're off to a good start because you have at least one grease gun more than your clients.

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There are a number of publication on friction and companies who measure the conditions. A comprehensive report was published by the USA nuclear energy department who compare measurements of many rope sizes, sheave sizes and rope composition to determine rope failure under various loads, rope wrap and sheave sizes. The report is exhaustive with many hundreds of fatigue to failure tests.

Tension Member Technology is based in Los Angeles is one of the firms who have a battery of sheave and rope test facility that can quantify certain aspects of fatigue to failure of many types of rope construction for both steel and fabric - ship lines, balloon tethers, crane ropes, etc. Go to their website. They are one of a number of USA, South African, UK and European installations that have full laboratory test regimes to quantify many of the discussion points raised here. ■