Shoe presses

The increase in the rate of pulsing is as follows. When you apply force to the brake pedal, the shoes press against the dmm for 1/2 second, then the shoes move away and release pressure for 1/4 second, then they press again for 1/8 second, then they are off for 1/16 second, and so on. It turns out that this infinite series (1/2 + 1/4 + 1/8 +. . . ) adds up to 1. Therefore, at the end of 1 second, the brake shoes have switched an infinite number of times because 1 second = 1/2 second + 1/4 second + 1/8 second. 

Steve Frye from North Carolina suggests that the probability of finding the brake engaged is 2/3. For example, the first time the shoes press against the drum they press for 1/2 second, then they move away and release pressure for 1/4 second. The brake continues to spend twice as much of the remaining time engaged as disengaged. 

The one-nip shoe press reducing investment costs, space requirements and operating costs to a minimum... 

In fast running machines the dynamic heat build up in the cover limits the application of elastomeric materials. Polyurethane polymers, based on special formulations with excellent dynamic properties, ensure good performance in these applications with minimal heat build up. An example of such an application is the substitution of grooved steel rolls by elastomeric covers in shoe presses. 

All press nips in modern paper machines are single or double felted. Shoe presses are mainly double felted. The pick-up felt is in the first press felt position it has to transfer the wet paper web from the forming wire to the press section. Whilst the loads of the press nips increase from press position to press position of the press section, the diameter of the felt fibers on the paper side surface decreases. Changing from a coarser press felt surface to a finer one enables the transfer of the paper web, due to the increased adhesion by capillary forces and larger contact area. 

Transfer belts are a specialty used in the second bottom position of a double shoe press replacing a press felt These transfer belts guide the paper web from the press section to the dryer section. The requirements transfer belts have to meet are as follows ... 

Nowadays, the dry content after a shoe press section reaches about 50 to 55%, depending on the product and the raw material used. Because of these high dry contents, less thermal energy is required in the dryer section, and the resulting increased web strength results in fewer breaks. 

Fig. 6.47 Shoe press consisting of a top roll with sleeve and concave shoe and a bottom roll with iron shell supported by hydrostatic bearings (source Voith).

Shoe presses have been standard in press sections for board and packaging grades since the 1980s. Later, they have also become state-of-the-art presses for graphic paper machines. Today modern press sections of high-speed paper machines consist of only two nips. They have for instance two double-felted nips or one double-felted nip followed by a second nip with a felt and a transfer belt... 

Fig. 6.48 Pressure distribution in the machine direction in a conventional two-roll press nip and a shoe press (source Voith).

Fig. 6.51 Shoe press in tissue production (source Voith).

Shoe presses are also used in tissue production. Here the advantages are higher bulk and sheet dryness after the press. In this application the shoe press replaces the suction press roU of a conventional machine acting against the Yankee dryer (Fig. 6.51). 

This method is also a combination of pressing and drying. The process takes place in a press nip (for instance with a shoe press) where one surface, which is in direct contact with the web, is heated. The other web side is in contact with a felt The wet web is compressed and thus mechanically dewatered. The vapor generated at the hot surface pushes the water through the compressed capillaries towards the felt and finally the generated vapor can flow freely through these channels. This kind of process is stiU in development... 

Atkins, Single.nip shoe press the single choice. Solutions )uly 2002, p.40. Augscheller, Anwendung eines Ein-Nip-Pressenkonzepts, Das Papier 2004, T203 pp. Hansen, Precision engineering for the press section, ipw 2003, 5, 22. 

T. C. Thorstensen, Fundamentals of Pollution Controlfor the Eeather Industy, Shoe Trades Publishing Co., Cambridge, Mass., 1993 K. H. Gustavson, The Chemistry and Keactivity of Collagen, Academic Press, Inc., New York, 1956. 

Tablet Press. The main components of a tablet compression machine (press) are the dies, which hold a measured volume of material to be compressed (granulation), the upper punches which exert pressure on the down stroke, and the lower punches which move upward after compaction to eject the tablets from the dies. Mechanical components deflver the necessary pressure. The granulation is fed from a hopper with a feed-frame on rotary-type presses and a feeding shoe on single-punch presses. A smooth and even flow ensures good weight and compression uniformity. Using the proper formulation, demixing in the hopper is minimized.

For applications requiring two different compounds, such as a double hardness sealing ring or a two colour rubber shoe sole, some machines have two injection units. One injects into the bottom/centre mould plate and the other into the parting line. Two injection units on a singlestation vertical press are used in combination with twin rotating cavity plates. 

The injection-compression process is an important variation in which rubber is injected into a partly closed mould under low pressure used for moulding-on of rubber soles to footwear uppers. Rubber is injected into a gap between the shoe upper and the sole plate, which is held 3-4 mm wider than it would be in its final position. After injection at the first mould station of a multistation machine the sole plate is closed in the manner of compression moulding at the second mould station and rubber is pressed along the sole and over the toe cap. The importance of the process is that it becomes possible to mould without using normal high injection pressures which tear and distort canvas, leather or synthetic uppers. 

Details are given of the E266-4 injection press developed by Main Gronp for the production of shoe soles from EVA and crosslinked EVA foams. 

SBR is now used as an adhesive in carpet backing and packaging. Neoprene (polychloroprene) may be blended with a terpene or a phenolic resin and used as a contact adhesive for shoes and furniture. Contact adhesives are usually applied to both surfaces, which are then pressed together at a later time. 

Wet nitrocellulose can be pressed down manually in the kier by workers dressed in protective clothing, rubber shoes and woollen clothing for entering the boiler. In this way, as used in France, the loading density can be increased to 100 kg/m3. 

In addition, the rotary-platform, double-head abrader is specified for coated fabrics58 and the NBS abrader for shoe soles and heels59. The NBS abrader uses rotating drums with abrasive paper wrapped around them onto which the test pieces are pressed by means of levers and weights, but there is no provision for traversing the test piece across the abradant as in the DIN machine. 

---