Dear Ghandoko:

The terms "half trough" and "full trough" with regard to terminal pulley positioning refer to the height of the pulley in relation to the trough idlers. The full trough position, which is the normal and preferred one, has the top of the terminal pulley in line with the top of the center idler of the trough set. The half trough position raises the pulley by one-half of the trough height (one half of the distance between the centerline of the horizintal idler and the upper end of the angled idlers.

The half trough position is used when it is discovered that the transition zone is too short. A short transition zone can be responsible for damage to the belt, pulley, pulley lagging, and the first few trough idlers. The half trough positioning is an attempt to redistribute the stresses so that the belt and other components do not wear out too quickly. It would be better to reconfigure the conveyor to allow for a proper transition distance if at all possible (and use the full trough position).

The transition length required will vary depending on the construction of the belt carcass. Generally, a low modulus belt requires shorter transition distances. The belt manufactire can advise on the transition length required for the various trough angles.

Although I do not like the use of half trough pulley positioning, it is better than replacing the belt, pulley, lagging, and idlers sets on a continuing and frequent time frame.

Regards, ■

Just a comment:

Often the terminal pulley center is raised about 1/3 not 1/2 the trough shape to:

a) minimize the belt edge tension wrt to the center tensions

b) minimize transverse bucking at the idler junction that can lead to belt failure at the junctions - usually with worn covers transverse rigidity is not sufficient to restrain buckling which leads to carcass failure.

The stress pattern can be evaluated using the area moment method. Dave Beckley published this in Trans Tech Bulk Solids.

Be careful, pushing the belt out of iits natural plane causes the material to be lanched over the pulley making the prediction of the trajectory difficult. Further, this disurbance can lead to excessive rock motion at the discharge increasing spillage around the head end.

Lawrence Nordell

President

Conveyor Dynamics, Inc.

1111 West Holly St.

Bellingham, WA 98225

USA

ph 360/671-2200

fx 360/671-8450

email nordell@conveyor-dynamics.com

www.conveyor-dynamics.com ■

The conveyor transition must be understood in terms of basic mechanics. A troughed belt has a neutral axis which passes through the C G (center of gravity) of the carcass cross-section. Tension along the neutral axis produces no net uplift force or additional downward force on the idlers. The neutral axis is approximately (actually a bit less than) at 1/3 of the trough depth above the center roll, for a equal roll troughing configuration. A transition where the top of pulley lines up with the top of the center roll is equivalent to an abrupt downward turn in the beltline. The last idler will be overloaded by the product of the belt tension and the sine of the abrupt angle change. Where the pulley is placed above the (top of center roll) beltine, 1/2 of the trough depth, there will be an uplift force calculated in the same way. Often, at the discharge (high tension end), in high tension belt applications, the belt will lift off of the idlers. Rather than lower the pulley to the neutral axis the installer typically shims the idlers up to the lifted belt line.

Dos Santos Inernational typically designs a conveyor profile with reference to the neutral axis belt line rather than the traditional top of center roll belt line. This practice completely elliminates the problems cited.

Basic understanding of this phenomenon will hopefully help you avoid the concequential problems.

Joe Dos Santos

jds@dossantosintl.com

www.dossantosintl.com ■

One additional thought on this subject, a half trough belt will be extremely difficult to seal if this is used at thje tail pulley. The belt line will move away from the normal path and make the dust seal not effective. ■

Referring to the points mentioned by Mr. R J Morrish, following are the comments.

While adopting the pulley rise compared to general belt line, the belt profile (concave curvature) is also analysed at this location and idlers are placed vertically and horizontally according to the calculation. Also, belt stability in transition zone is checked and the proper design takes care that the belt lift will not reach to the skirt board edges.

Regarding the rock falling due to unsupported belt is not correct. In fact, the belt is adequately supported by idlers of different troughing angle in the transition zone, this refers to only belt support and lump falling or rolling from belt is a different issue. In general, the belt will pass through zone very quickly and lumps falling will be an exceptional situation, which is to be taken care by other means i.e. to have extra margin in material cross section capability.

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 5882916 ■

Hello ghandoko,

The earlier respondents have already given certain information. I will like to add the following.

If the conveyor layout permits, one places the terminal pulley top point on straight line drawn touching the top of adjacent central rollers of troughing idler sets. In this case, transition distance from flat to trough is the maximum.

One can reduce the transition distance by placing the terminal pulley comparatively at higher level (in relation to general belt line). This is called raising of terminal pulley. The magnitude of raising will be of necessary value and it is not necessary to be only 1/2 or 1/3 of trough depth. Of course there are limitations beyond which a pulley cannot be raised. The raising can be few inches / centimeters. Sometimes even raising just by two inches will make remarkable effect on transition distance. This is only to give an idea about the magnitude of raising.

The required raising of the pulley is decided by specific calculations in conjunction with belt specifications & data.



Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyor.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 5882916 ■

A further comment:

I don't understand Mr. Mulani's above qualitative comment leading some to believe raising the pulley a few inches will do great things. This is true when you are working with belts less than 36 inches and 25 degree trough angles. I agree with Joe, placing the pulley's elevation over the theorectical work point is to: 1) minimize belt edge stress which may allow, 2) a reduction in the idler transition distance.

Raising the belt to the half way position makes matters worse with respect to minimizing belt tensions and other detrimental effects.

Another factor neglected in the above discussions is the significant change in ore trajectory with the elevated pulley. This will be much harder to define with the elevated pulley. The elevated pulley leads to what some call the "ski jump effect". The ore is lanched to an equivalent higher belt slope than is obvious with the in-line general slope. A hyperbolic equation is used to define this shape as the ore leaves the pulleys tangenetial release point. The simple SAG equation may get close depending on belt tension.

Lastly, the raised pulley unloads theforce on in-line transition idlers. Proper postion, to carry equal load, is not obvious. Most engineers get it wrong which leads to the comment of overloading.

Lawrence Nordell

Conveyor Dynamics, Inc.

website: www.conveyor-dynamics.com ■

Hi Guys,

Further to the discussion on this, there are 2 trains of thoughts.

A raised pulley postion leads to shorter transition disctances, refer calc by Dave Beckley et al.

However, at the head end and with large lumps, U could have the possiblity of belt carcass damage due to rock loads from the step change - plus trajectort changes.

And, if U have flat belts, raised pulley will not drain rainwater out at the tail or other station.

Also, a lift at the tail pulley whihc is close to the loading skirts can cause wear due to belt lift as the belt cover wears/starting condtions etc.

We prefer no lift at tail pulleys and lift at head ends.

Thanks

James Morrish ■