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Small diameter PC Strand

It is well known in the industry that PC strand in small diameter size is not a product that the producers like to make or are even willing to make since with the current equipment the efficiency is very low and the selling price does not make up for the loss of efficiency.

Typically, a line producing 12.7 mm PC strand, can make strand in excess of 6 tons per hour, while the same line, producing 6.85 mm strand will not exceed 1 ton per hour at the most.

Small size PC strand however is requested by the market, and many of the customers require the producers to supply the PC strand smallest sizes if they want to supply the larger and more profitable sizes as well.

In several countries there is a market for 3 wire stabilized strand, and this product as well has the same characteristics of small diameters PC strand: it is not produced very efficiently.

We were asked by several PC strand producers to address this issue and propose some alternative equipment that would be able to produce 3 wire and 7 wire strands in a more efficient manner that what is currently being done in the industry.

For this purpose, we have developed a new strander specific to these products that would have a production speed double than a standard pc strand line but still be capable of using 1250 mm spool as feed spools, so that the loading time would be reduced to a minimum.

The new strander takes advantage of the double twist technology, so that with every revolution of the bows, two twists are obtained.

The strand take-up spool was designed to hold 4.4 tons of strand as the typical package weight of this product is about 2.1 tons. The take up spool has a diameter of 1250 mm, allowing the strander to rotate at a speed of 800 rpm, equivalent to 1600 twists per minute which means that 6.85 mm 7-wire strand can be produced at a linear speed of 150 m/min, against as speed of 75 m/min with any other standard skip stranding line.

Obviously, the stranding process and the stress relieving process must be separated into two distinct steps but this can be an advantage as one stress relieving line can be designed fast enough to support two such lines.

The DTUF 1250/7-PCS strander

The DTUF-1250/7 - PCS, is a double twist stranding machine that is composed by the following elements:

The 7 motorized payoff units are suitable for 1250 mm diameter spools with a capacity of 4 tons each. The motors are used to apply the correct back-pull to the wire filaments and synchronize the speed with auto diameter system that does not require any adjustment. Having motorized payoffs gives the following advantages:

  • No requirements of brakes and brake pads that require regular replacement.
  • Adjustment of wire back pull fully electronic through keyboard.

Following the payoffs, the line is equipped with a large overtwister. The purpose of this unit is to pre-form the cord with the correct length of inner wire and outer wires.

As the inner wire is shorter than the outer wires, the overtwister makes sure that the inner wire is not too long so that in the twisting process it is exchanging position with any of the outer wires.

The over twister closes the strand and re-opens it at the same time, feeding the twisting machine with the correct length of wires composing the strand.

The take up unit or strander is a double twist machine equipped with carbon fiber bows. The wires enter through the shaft and are directed to the bow which brings the strand to the other side of the strander. A large pulley on the shaft directs the wires that have now formed into a strand on the double pulling capstan which pulls the strand through all the line. The strand is then redirected through a second overtwister that closes the strand completely and then onto the spool where the strand is collected. The spool has a capacity of 4.4 tons so that it is possible to make two packages of 2.2 tons each to be sent to the customer.

From the twister the strand is directed to the rotating take-up where the final formation is given and where it is wound on the take-up spool.

The take-up unit consists of an electro-welded metal structure on which are mounted all the working parts; including the cradle with the spool and the drive motors, the flyer, as well as the spool lifting group.

The take-up main shafts and bow are driven by a variable speed motor and power is transmitted by toothed belts and pulleys. The machine is braked with pneumatically operated disc brakes, activated by a solenoid valve interlocked with a compressed air regulator.

The flyer consists of bows with an aerodynamic design. The path of the strand on the bows is defined by bushings that assure easy and homogeneous passage of the twists.

The flyer shafts are hollow and rotate on load and thrust bearings that are grease lubricated. A centralized, manual or automatic feed lubricating system is optional. Deviations inside the shafts and on the cradle are also obtained with large diameter pulleys.

The cradle is a portal type electro-welded steel structure where pulling capstans, over twister, straightening unit, traversing unit and spool tailstocks are mounted. The spool is horizontal axis parallel to the machine axis.

Take-up pull is provided by two haul-off grooved capstans. The haul-off capstan is driven by planetary type reduction gears. The strand pulled by the first grooves of the capstan passes through the wire straightener and then goes back to the second series of grooves and is finally wound on the spool.

Due to the small difference in diameter of the first and second series of grooves this system allows to perfectly check the cord stabilization through adjusting of its mechanical tension on his path through the over twister and wire straightener.

This peculiarity of the machine combined with other high technological constructional details allows the perfect formation of the cords free from any waves, warps and residual torsions.

The great flexibility to adjust the various machine components allows the construction of strands with different characteristics according to the customer’s needs.

The capstans are motor driven, and this allows to set the desired lay length by setting the lay value and lay direction on the video keyboard.

For all the stranding machine a twist setting unit (TSU) (over-twister) is foreseen.

The TSU is driven by a separate A.C. motor with frequency inverter and synchronized with the main motor in order to have all the time the same speed ratio. The ratio can be set on the video keyboard.

The spool tailstocks are activated simultaneously to open by air pressure and close by pre-loaded springs. Closed position is automatically detected to prevent accidental opening.

The spool is loaded and unloaded with hydraulically controlled lifting platform installed in a small pit under the cradle. The spool can be lowered to the pavement level. No need of special foundation, just a small pit 200 mm deep. The spool is motorized by a separate A.C. motor frequency inverter driven.

The traverse system screw type is also driven by a separate A.C. motor.  The traverse lay as well as the inversion points can be changed even during operation.

The drive of all the motors inside the cradle is carried out by collectors and brushes that are mounted at the ends of one of the torsion shafts.  The number of power and signal brushes is reduced down to the minimum using Bluetooth for signal transmission. Collectors and brushes are protected from dust.

The take-up structure is completely enclosed in an accident prevention/sound-absorbing cabinet which is provided with forced ventilation, internal LED lighting and a large-sized door for easy access to the inside. The opening and closing of the door are pneumatically activated.

Electrical equipment

The electrical equipment is contained in a metal cabinet and cooling can be by forced ventilation or by water air heat exchanger or by air conditioning. The doors are lockable and cannot be opened if the main switch is on. A signal warns the operator if the doors are open.

The electrical equipment consists of the power and control components for the digital drives of the motors.  The drives are totally controlled and reversible, with personalized setting circuits that can be removed and transferred without re-aligning the machine.

A solid memory PLC is foreseen to control and synchronize all the operating parameters of the machine.

A separate operator control desk includes an LCD keypad display connected to a micro-processor, with which all the machine settings are made (motor speed, meter counter length, direction of Stranding lay, lay length, etc.). The control desk also includes all the buttons, and signal lights to operate and monitor the machine. A remote assistance system is integrated in the electrical cabinet to facilitate remote trouble shooting if necessary.

Machine foundations are not necessary excluding the small pit for the lifting platform.