GB2283729A - Web turning apparatus - Google Patents

Abstract

In web turning apparatus, including a tube 1 around which a web may pass to change its direction of travel, the tube including a plurality of pairs of parallel elongate Coanda nozzles 25, each lying substantially parallel to the tube axis, and a Coanda surface 27 between each pair of nozzles so that air fed through the nozzles passes across the surface by the Coanda effect to converge to form an air cushion, air is fed to the tube interior from a duct 17 via a diffuser 15 and apertures 21. The tube 1 may include internal support structure comprising a shaft 9 mounting discs 7 apertured at 23. <IMAGE>

Description

WEB HANDLING APPARATUS Field of the Invention The use of air flotation systems to handle webs is well know. However, there is a growing demand for web handling apparatus to fit into ever more confined areas. One attempt to achieve such apparatus is detailed in the applicant's copending UK Patent Application No 9209575.1. However, in practice, this has a minimum diameter of 200mm. Applications are emerging for web handling in even more confined areas down to lOOmm diameter. It is the purpose of this invention to satisfy that demand.

It is clear that the constraints on size could be overcome by a solid bar or tube roll or, in fact, a perforated high pressure "greasing roll".

However, there are applications within the industry where it is imperative that the web handling is conducted such that surface contact is eliminated, eg in the processing of film or any product where surface scratching/blemishing is undesirable or the handling of newsprint with modern colour printing.

In these applications it is clear that a contact roller could not possibly be used. The high pressure "greasing roll" operates at very low clearances of typically 0.025cm, thus the chance of particulate entrainment effecting a bridge between the greasing roll and the product is high. Once this has taken place there is the risk of surface damage. It is also to be noted that the greasing roll tends to be dedicated to operate over only a limited range of tension, thus a certain amount of flexibility in operation is lost.

The purpose of the invention is two-fold.

Firstly, it must fit into the available confined space.

Secondly, it must perform well when handling these sensitive products.

To satisfy the performance criteria the unit must operate in a truly defined contact-less manner with a known constant across the width flotation height. This flotation height must, of course, be great enough to ensure no risk of surface damage.

The performance of the unit related to web handling must be independent of the process variables of web width, type, speed, tension etc.

Summary of the Invention According to the invention there is provided web rotating apparatus, including a tube around which a web may pass to change its direction of travel, the tube including a plurality of pairs of parallel elongate Coanda nozzles, each nozzle lying substantially parallel to the axis of the tube, and the profile of the tube defining between each pair of nozzles a Coanda surface so that air fed through the Coanda nozzles passes across the Coanda surface by the Coanda effect to converge to form an air cushion; means for feeding air into the centre of the tube; and, diffuser means for ensuring a uniform air cushion.

The Invention into Practice The invention will utilise a scaled version of a typical Coanda air bar. The Coanda nozzle surfaces will be formed by accurately machining the Coanda nozzle profile into a thick walled tube. The machining length will not, of course, be continuous for, if it were, the tube would lose its stiffness and integrity, hence areas of full tube wall will be left as unmachined "lands" which will act as stiffening ribs. The nozzle tube is further stiffened and supported by internally mounted perforated discs secured to a central through shaft fixed into the end plates. The Coanda nozzle is mounted onto a back "air feed duct". This back feed duct has secured between its side faces, at just below nozzle tube level, a diffuser sheet which has the double effect of giving stiffness to the back duct and equalising the air flow to the nozzle tube.

The air flow to the unit, ie to the back feed duct, can be introduced anywhere along its length, but preferably at one end.

Features that are not essential: Perforations within the Coanda Nozzle Face Because the actual distance between the Coanda nozzle slots is small these perforations are not essential. However, as mentioned earlier, they do assist in stabilising and feeding the cushion.

Nozzle Tube Support System Dependent on the length of the unit, ie less than say 800mm, the unit itself will have sufficient inherent stiffness not to warrant the fitting of the internal support system.

In summary the invention must have the following features: a) Outside nozzle diameter typically to be in the range 100250mm.

b) Profile on the outside diameter of the tube to be the Coanda nozzle profile.

c) Method of ensuring uniform air cushion ie diffuser/ perforated discs.

d) Back duct to supply the air flow.

A further development of the invention is to utilise aluminium extrusion technology to generate the required Coanda nozzle profile as opposed to machining. Essentially the two units are the same except that the upper portion of the machined tube is replaced by an extrusion, as detailed in Fig 4. The remainder of the invention is unchanged apart from some slight mechanical detail with reference to the manner in which the nozzle system is affixed to the back duct.

Description of the Drawings Fig 1 Cross-sectional end view of the mini roll.

Fig 2 Method of supporting and maintaining the integrity of the nozzle tube.

Fig 3 Cut-away section of the nozzle tube.

Fig 4 Cross-sectional end view of the roll comprised of aluminium extrusion.

Description of the Preferred Embodiment The preferred embodiment is as pictorially represented in Fig 1. The unit comprises of a Coanda tube nozzle (1) the nozzle profile (3) of which has been accurately machined. The nozzle tube owes some of its strength and integrity to the unmachined portions of the tube known as the lands (5). The strength and integrity of the nozzle tube is further enhanced by the tube support system as detailed in Fig 2, ie beneath the lands (5) there are situated perforated discs (7) which are securely mounted onto a central through shaft (9). This shaft (9) is fixed between nozzle tube end caps (11) which also have the additional function of sealing the nozzle tube (1). The nozzle tube (1), complete with the tube support system Fig 2, is mounted on to a back air feed duct (13). The back air feed duct is stiffened by virtue of a diffuser (15). This diffuser (15), as well as adding stiffness to the back duct, also introduces a pressure drop to the air flow thus creating a more uniform air flow to the nozzle tube chamber (19).

Operation of the Preferred Embodiment The air stream is introduced to the unit via the connection end of the back feed duct (13) [connection flange not shown].

The air stream "fills" the entire back duct chamber (17). On its passage through the system the air stream then encounters the diffuser (15) which, by splitting the air stream into a plurality of smaller air streams and thereby introducing a pressure drop, has the effect of creating a substantially uniform air flow across the diffuser (15), ie from the back duct chamber (17) to the nozzle tube chamber (19). The air flow is introduced to the nozzle tube chamber (19) via inlet apertures (21) at substantially uniform air pressure along the nozzle tube (1) length. Once inside the nozzle tube chamber (19) a final period of air pressure equalisation is brought about by minimal exchange of static air pressure across the pressure equalisation vents (23) within the support discs (7).

The air is fed from the nozzle tube chamber (19) through the Coanda nozzle slot (25). This passage of air through the Coanda nozzle system (3) forms an air cushion by virtue of the Coanda effect on which the web to be handled is supported.

Now, because the pressure of the air delivered to the Coanda nozzle system (3) along the length is substantially equal, the magnitude of the Coanda effect and, hence the supporting cushion, is substantially equal. Also the Coanda surface (27) may include a row of perforations (29) arranged parallel to and equally spaced between the Coanda nozzle slots (25). These perforations (29) assist in the formation and stability of the air cushion. If the angle between the direction of the web onto the unit and the direction of the web off the unit is labelled as the angle of wrap, then the number of Coanda nozzles is adjusted to suit any specific angle of wrap.

However, the preferred embodiment will be an angle of wrap of 1800.

Because the tension that the unit will support is proportional to the magnitude of the air cushion generated, by selecting a fan with a volume and pressure duty that will satisfy the operating conditions at the maximum tension and by trimming the fan duty for lower tensions either manually or by automatic control the unit will operate independent of tension and at the desired web flotation clearance.

Claims (1)

1 Web rotating apparatus, including a tube around which a web may pass to change its direction of travel, the tube including a plurality of pairs of parallel elongate Coanda nozzles, each nozzle lying substantially parallel to the axis of the tube, and the profile of the tube defining between each pair of nozzles a Coanda surface so that air fed through the Coanda nozzles passes across the Coanda surface by the Coanda effect to converge to form an air cushion; means for feeding air into the centre of the tube; and, diffuser means for ensuring a uniform air cushion.