OVEN CONVEYOR BANDS
(from Baking Process and Engineering Manual)
There are three major types of oven bands for conveying the dough pieces through the oven. Each type of band has particular baking characteristics in terms of heat transfer and physical performance and therefore has a particular range of applications.
1. Open wire mesh bands
These are the most versatile and widely used oven bands. They are suitable for crackers, hard biscuits and rotary moulded products, in fact all types of biscuits except soft cookies with high fat contents, where the dough will flow into the holes in the mesh. The wire mesh bands allow convective air to pass through, heating the base of the dough pieces.
These wire mesh bands are normally made from carbon steel wire coils which are linked together. The links give a flat surface to the band and good flexibility. Alternative materials are available for special applications, for example stainless steel wire mesh bands may be used where the baking process has very high humidity for products such as cheesecake.
The most popular bands are often referred to as Z47 and Z47R. These designations are from a European supplier, Produits Trefiles de la Bridoire (Agrati Group). Similar types of high quality wire mesh band are F4102 from Steinhaus Gmbh and U66-48-1718 from Ashworth Inc. Cambridge Engineered Solutions supply balanced weave baking belts.
Steinhaus F 4012 even thickness, accurate width, welded edges
Wire mesh bands: skid bar supports
Wire mesh bands are successfully supported in the baking chamber by cast iron skid bars. These bars are placed inside the baking chamber with a pitch of 0.8 – 1.2m. They are supported in frames and are removable for cleaning. Apart from cleaning, which is infrequent (normally annually), skid bars do not require any maintenance.
Skid bars from Sandvik Process Systems
The skid bars are cast from iron with a graphite content which reduces friction. The material and casting are critical to the performance and the skid bars are supplied to suitable specifications, for example International ISO/DIS 185 Grade 200 / 250. The chemical composition, casting process, shape of the mould and cooling rate are important to achieve the optimum low friction and durability of the skid bars in operation.
The surface of the skid bar has a microstructure of a pearlitic matrix with evenly spread type A graphite flakes to ASTM A247 standard with no hard particles. Maximum allowed amount of ferrite is 10%. No ledeburite is allowed. The sliding surface of the skid bar is machined after casting by removing the surface layer (1.0mm), which contains ferrite and would result in scratching of the band. The final surface should have a roughness of Rmax 3.0µm
The installation of the skid bars is critical to good band tracking. The bars must be level and at right angles to the direction of band travel. Levelling is best done optically with a laser instrument.
Return band supports
On the return below the baking chamber, the band is supported on rollers. A proportion of these, usually one in three, may have spring loaded guide discs. These are very effective in keeping the band in its central position.
Wire mesh oven band cleaning
Oven band cleaning is critical to the quality of the biscuit and the oven performance. We strongly recommend a band cleaner which operates all the time during baking, ensuring that there is no build up of carbon, which is difficult to remove.
The Baker Pacific oven band cleaner consists of two independently driven wire brushes for cleaning both sides of the band. The unit is designed for a continuous, gentle cleaning action to maintain the band in good condition. The top wire brush, driven by a fixed speed motor gearbox cleans the inside of the oven band. The lower wire brush unit cleans the outside of the oven band. It is mounted on a trolley for removal to the side of the oven. Guide rails are provided for the trolley with stops and locking handle. The brush is raised or lowered to the working position by a lever and the brush pressure is adjustable.
Maintaining a clean band is important, particularly when baking products with sugar topping, when preventing contamination is vital. When baking products with toppings a good system of recovery for surplus toppings is essential to prevent toppings dropping on the oven band. A long wire-mesh panning conveyor is recommended with a level transfer of the dough pieces to the oven band. The panning web should an open mesh band (enrober wire-mesh) and be around 2 – 3m long. The end transfer roller or nosepiece is of small diameter to allow a level transfer of dough pieces to the oven band.
For wire mesh bands which have become dirty, some hard carbon deposits may be removed by heating the band to 400oC, when the hard carbon deposits will break and can be removed by the brushes. It is important that the band is heated evenly and only sufficient temperature to carbonise the debris is used. Uneven or excess heating will distort the band.
Soft deposits may removed by steam cleaning with an industrial cleaning fluid. Suitable drainage is required. After cleaning, the band should be dried and oil (soybean oil / coconut oil) applied to prevent rusting.
Wire mesh oven band tracking
The position of the oven band edge is constantly monitored by vertical rollers on each side. The rollers are supported on a bar which follows the movement of the band. The bar carries a Euchner trip rail which engages a multiple limit switch. If the band moves up to 10mm it will automatically actuate a pneumatic cylinder which moves a tracking roller below the band. The oven band will always try to leave a support roller at 90 degrees to its angle. Therefore by altering the angle of the tracking roller, the band is pushed back to its central position. If the band moves more than 25mm, then the drive will automatically stop. These settings are adjustable.
Auto tracker at feed end
The tracking assemblies are mounted just before each end drum to guide the band onto the drum in a central position. The delivery end tracker is on the top and the feed end tracker is on the return band at the Abottom of the oven. The trackers may have several rolls on a single frame operating together.
Auto tracker at delivery end
Vertical guide rollers are also used inside the baking chamber. Typically there is one assembly in each oven zone adjacent to the inspection door. These rollers are pivoted and may be spring loaded.
An alternative method of band tracking moves the delivery end drum to skew it. It is pivoted at one side and moved by a pneumatic cylinder at the other side to alter its angle to the direction of travel of the band. However this system is not recommended by some band manufacturers as it alters the distance between the feed end drum and the delivery end drum on one side, creating a tight side and a loose side of the band. Oven band manufacturers generally recommend that the drums are set exactly at right angles to the direction of travel of the band.
Joining wire mesh bands
To join the ends of a wire-mesh band, two spiral joining wires are used. These are inserted from the side of the band. After insertion, the spiral joining wires are hammered flat to provide a secure and almost invisible joint.
Pictures from Steinhaus GmbH
2. Heavy mesh bands
These are tightly woven bands with a “herring bone” pattern presenting a continuous flat surface. They are relatively heavy and have a high heat mass. These bands are pre-heated and will conduct heat directly into the base of the dough pieces. They are widely used, particularly in North America, for the baking of soda crackers, but are also versatile and may be used for all types of biscuits, except high fat cookies.
Cambridge Engineered Solutions: CB5 Compound Balanced Weave bands
Heavy mesh band supports
Heavy mesh bands are supported on rollers, which will carry heavy bands with low friction. The design of the supports and adjustment for levelling and the specification of the bearings are critical as the rollers operate at baking temperatures. If the rollers seize, they will quickly cause wear to the oven band. The ends of the rollers are therefore exposed for inspection and high temperature bearings must be used. The bearings are mounted outside the baking chamber and are insulated from the chamber.
Roller bearing assembly SKF type Y-V228 for operating temperatures up to 350ºC
Heavy mesh band tracking
Heavy mesh bands are tracked by vertical side rollers positioned before each drum. These rollers are not intended to force the band into position. They do limit side movement and act as sensing devices to indicate tracking problems.
The band control units should be located at each end of the oven before the terminal drum. The distance between the control unit and the drum should be 3 x band width. At this distance the force required to move the band to its central position on the drum is small.
Joining heavy mesh bands
The bands are joined by inserting the correct number of connecting wires, depending on the type of mesh. The crimped connectors are inserted and seated so that the weave is flat. The connecting wires extend each side by 2mm each side and are fastened to the end spirals by a simple weld.
3. Steel bands
These bands are made from thin (usually 1.2mm) carbon steel and are very suitable for soft doughs, which flow on the band when heated. They are used for all deposited cookies and cakes, such as layer cake which are baked directly on the oven band. Traditionally steel bands are also used for baking “Marie” biscuits.
Cookies deposited directly on a steel band
Steel band supports
Steel bands are successfully supported on skid bars as described above. Skid bars are manufactured and recommended by Sandvik. In the case of steel bands, the skid bars are 75 – 100mm narrower than the band. The pitch of the skid bars should be 0.8m – 1.0m.
Special soft graphite skid bars may be used at the feed end of the oven. Approximately 4 graphite skid bars are used for a 30m long baking chamber. These skid bars lubricate the inside of the band and reduce friction. The soft graphite skid bars apply graphite in an even, continuous, automatic way. They provide the following advantages:
- There are no limitations on speed and they are ideal for heavily loaded bands
- The band does not need additional lubrication start up or during operation
- Band life is increased
- Can be used in conjunction with cast iron skid bars to help to reduce friction and eliminate any slip-stick problems
Hard graphite skid bars are also available. These provide a long life at high temperatures. They may be used in place of cast iron skid bars.
Joining steel bands
Steel bands may be riveted and Sandvik supply riveting kits including the tools and instructions. However a much better method is welding, which provides a completely smooth surface to the join. Welding is a specialist operation carried out by the band supplier with specialist equipment for welding and tempering the joint. Sandvik South East Asia PTE Ltd. provide equipment and specialist engineers. sea.sps@sandvik.com
Steel band cleaners
Steel bands may be cleaned by rotating brushes (as described above for the wire – mesh bands) assisted by scrapers.
For soft, sticky products such as layer cake, steel scrapers are used. Up to 3 rows of spring steel scrapers in a staggered formation are mounted on the adjustable frame. The frame is raised by a lever to scrape the band. The pressure is adjustable.
Steel band scrapers on a layer cake oven
For very sticky products hot water washer units may be used. These are located close to the delivery end of the oven, where the band is still hot. The washer may be used continually or intermittently as required. The water is sent to drain. After washing, the band may be oiled to prevent rust.
Steel band greasing
At the feed end drum, the band may be greased to prevent the dough sticking. The grease is deposited from a hopper onto soft pads which are mounted on a reciprocating arm. This moves across the band, back and forwards to smear an even coating of grease on the steel band.
Steel band tracking
Spring loaded guide rollers provide a robust and safe way to keep a band running within acceptable side movements. A pair of side guide rollers may be installed in each oven zone.
Sandvik side guide rollers
An active band tracking may use tilting rollers activated by a band edge detection device. The tilting rollers are automatically raised on one side, creating a tight (higher tension) and a slack (lower tension) side to the band. The band will track towards the side with the lower tension.
Sandvik tilting roller tracking device
More technical information is available from www.processsystems.sandvik.com
References
Baker Perkins Ltd: “Cleaning wire-mesh bands”: http://www.bakerperkinsgroup.com
Berndorf: “Bake Oven Belts”: www.berndorf-band.at
Cambridge Engineered Solutions: http://www.cambridge-es.com
Euchner: http://www.euchner.co.uk
Jan-Ola Jonsson: “The Properties of Steel Belts”
Produits Trefiles de la Bridoire: “Wire bands”: ww.agrati.com
Sandvik Process Systems: “Product Information”: www.smt.sandvik.com
SKF: “ Product information”, www.skf.com
W.H. Smith: “Cleaning and Care of Oven Bands” (1967): Biscuit Maker and Plant Baker
Steinhaus Gmbh: “Rolled Baking Oven Belts”: www.bakingovenbelts.com www.steinhaus-gmbh.de
Yangzhou Jinrun Mesh Belt Manufacturing Co. http://www.wmbbelt.com
For enquiries and more information please contact Baker Pacific at bakerman@bakerpacific.com.hk
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