Hard to visualize what you are referring to. Typically a screw take-up has the pulley bearings moving on the screw rods as the rods are turned with the screw mostly enclosed. There are other cheaper types. But to try and answer your question it would be better to have the threaded portion of the take-up in tension rather than compression.

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Although intended for gravity take up the principle of rope wrap can be applied to screw take ups just as well.

Existing bearings are remounted in rope sheaves: one end of the rope is fixed; wraps 180° onto a fixed sheave and the standing part is ended with a forward facing swivel screw. There is no strut load on the screw thread and no protrusion behind the tail drum working envelope.

Both screws can be seen at the same time so it is easier to count the turns.

When originally developed, this system enabled the use of dead shafts, it still does. But if adapted to live shafts then the sheave is considerably smaller and the price will be correspondingly less. ■

If its possible to send me a detail or photographs this would make answering this much simpler. But i certainly agree that the tension would be better in tension.

Engineering Solutions are supplying Screw Conveyor for many different types of bulk materials and if you need any assistance please feel free to contact us.

www.engineering-solutions.co.za ■

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There are many different configurations of the screw take-up; the top screw (push and pull on the screw plus an eccentric load that is taken by racking of the blocks); the bottom screw (with similar implications); the center pull screw with the long tail; short center pull screw where the pulling blocks are relocated after the short screw runs out), compression screw with hydraulic assist. The reason that the center pulls are often preferred is because the top screw types are often not properly design for eccentric load and the strong back to make it right adds expense. All are manual take-ups that don't provide a means for knowing what the belt tension is (except for the hydraulic assist if you have a pressure gauge and equate the pressure to the ram load).

Why is it that short conveyors get a pass on the requirement to know that the tension is what we calculated and kept at that level?

We have for years offered what we call semi-automatic take-ups and compact automatic take-up for short conveyors where the traditional auto take-ups are too expensive and take-up too much space. In the former screw take-ups are mounted on a slide frame and butted up against compression springs of selected strength and stiffness constant, so the spring deflection equals tension which can be set and monitored. In the latter case short twin hydraulic cylinder have their displacement tied together and forced to be equal.

Joe Dos Santos ■

Hello Shri Mukul Khare,

Your query refers to pros and cons for the take-up screw under tension or alternatively under compression. The screw is primarily a rod. The long rod ability to withstand tensile force is quite high as compared to such rod under compression. This is so, because its ability to withstand compressive force gets reduced due to buckling effect. Conversely, if one uses long screw subjected to compressive force, the required diameter for the screw will be comparatively more.

Also for screw under compression, the manufacturing (fabrication) quality needs to be of higher order to avoid eccentric force (i.e. distance between actual line of force and screw geometric centre line) which would further aggravate the situation. The engineering books provide the solution for both the situations. The screw can be considered as free to flex at ends.

The relative merits / demerits are as below:

Screw under compression: It does not protrude beyond conveyor, but diameter will be bigger if take-up stroke is long.

Screw under tension: Diameter is less. Its ability to take force is not so sensitive to the fabrication precision. The screw protrudes beyond the conveyor. The remedy is to provide handrail so that one does not bump with the screws. Such handrails need to be of removable type to facilitate maintenance and convenience to use hydraulic tensioner for setting tension in the belt.

The hydraulic tensioner is an appliance to serve many conveyors, and is not part of conveyor. This can be attached between shaft projecting ends and frame, by suitable means. The hydraulic tensioner is often manual device to induce finite tension in the belt.

Take-up stroke for chain conveyors is quite small, so buckling of the screw is not the criteria and one often finds the chain conveyors having arrangement using screw under compression, avoiding protrusion issue.

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 ■