Printing machines: the case for high precision bearings
10 October 2015
Printing is an inherently precise process that places specific demands on the components that go into the construction of a modern printing machine. Steve Lacey outlines the factors to consider when specifying bearings for sheet-fed and web-fed printing machines.
Printing machinery bearings are primarily used in the main cylinders of sheet-fed and web-fed printing machines and presses. Due to their high load carrying capacity, rigidity, accuracy and precision adjustment capability, these bearings support the key requirements of printing machines – high productivity, low maintenance costs and highest possible print quality.
Bearings for printing machines are normally specifically designed for each application, which requires the bearing supplier to work in close partnership with the manufacturer of the printing machine. As a result, the bearings are precisely matched to the requirements of the customer. However, finding the optimum in terms of bearing technology and affordability is always a challenge.
Furthermore, due to a wide range of requirements from customers, standardisation of bearings for printing machines is only possible to a limited extent. Therefore, the range of bearings available comprises many different types and sizes. As well as traditional multi-row, high precision cylindrical rolling bearings, suppliers can also offer non-locating bearing units (with and without eccentric geometries), locating bearing units, polygon bearings, combined linear and rotary bearing units, and tapered roller bearings. Bearings are also generally available with or without seals.
Printing machinery bearings are typically specified for use in centre plate cylinders, blanket cylinders, impression cylinders and transfer cylinders. But whatever their location on the machine, bearings must fulfil a range of different requirements:
The accuracy of cylinder bearings has a direct influence on print quality. Printing machinery cylinders are closely synchronised - there must be no relative motion in either the radial or axial direction - so bearings must meet certain performance criteria in terms of freedom from clearance, rigidity and runout quality. In addition, it must be possible to move the plate or form cylinders axially in a controlled manner and, depending on the machine type, to achieve oblique adjustment (diagonal register function).
In order to perform printing machine functions such as on-pressure, off-pressure or compensating for different paper thicknesses, it must also be possible to change the centre distances of the cylinders in the printing press. Printing machinery bearings therefore have eccentric rings that can be swivelled through a specific angle reliably and with low friction. These types of bearings offer several advantages for the user:
-High accuracy: this is achieved as a result of the bearing accuracy (P5 or P4 tolerance class) and the bearing preload. As a result, the bearing design is clearance-free.
-High rigidity: the accuracy and combination of bearing components, together with bearing preload, provide high radial system rigidity.
-Reliable eccentric adjustment: a rolling element-based swivel bearing enables reliable, low friction, eccentric adjustment and prevents the possibility of jamming and premature wear.
-Low operating temperature: due to their design, the bearings run with low friction, minimising heat generation.
-Compact design: the units combine all necessary functions within a single bearing, providing savings in terms of individual components (reduced component count), simplified handling and logistics.
-Ease of mounting: as several functions are combined into a single bearing design, mounting and dismounting is simple and secure.
In most cases, determining the size of a particular bearing depends on the requirements for load carrying capacity, rating life and operational reliability of the bearings. In addition to these criteria, bearings for printing machines also need to provide high rigidity and be clearance-free.
In order to determine the rating life, the influence of preload in the bearing must also be considered. However, the rating life can also be influenced to a considerable extent by possible misalignment of the bearing journal/sleeve, the tolerances of the components, lubrication and contamination.
Printing machine bearings are normally designed for a rating life of at least ten years. Depending on machine activity, this is equivalent to between 40,000 and 60,000 hours for the bearing in a rotary printing machine. Bearings for sheet-fed offset printing machines are typically designed for an operating life of at least 200 million printed paper sheets. As these machines are used on a very flexible basis, they are often retooled to accommodate a new customer order and so are operated at widely varying speeds. It is therefore more appropriate to state the life as a number of printed paper sheets rather than in hours.
Rigidity, friction, lubrication and seals
The rigidity of the complete system is influenced not only by the cylinder and adjacent construction, but also to a large extent by the bearing design. Due to their larger contact surface area, cylindrical rolling bearings or tapered roller bearings have considerably higher rigidity than ball bearings. They can also be preloaded and operated with negative clearance. This results in reduced vibration, which in modern printing presses with no bearer rings is an important consideration.
Another important characteristic of printing machine bearings is low frictional torque and therefore low bearing temperature. In general, the operating temperature of the bearing should not exceed +60°C in order to prevent any possible negative effects on the printing process. Cylindrical rolling bearings have particularly low friction and so are advantageous when operating printing machines at high speeds. Frictional torque and bearing temperature are heavily dependent on factors, such as bearing type and size, speed and load, bearing clearance, lubrication, the mounting position and the mode of sealing adopted.
Lubrication has a significant effect on bearing temperature. Using a special smooth-running grease will result in significantly lower bearing temperatures thancan be achieved with oil lubrication. And if grease is the lubrication of choice, sealing is also simplified. However, the advantage of oil lubrication is freedom from maintenance, but there are disadvantages, including higher friction and bearing temperature, as well as the need for more costly sealing. Moreover, the cleanliness of the oil will have a considerable influence on the rating life of the bearing.
Generally, oil used in the drive of the printing machine is also used for lubricating the bearings and may be mineral-based or a synthetic gearbox oil of viscosity classes ISO VG 68, 100 and 150.
The function of the sealing system is to retain the lubricant in the bearing and prevent the ingress of contaminants and moisture into the bearing. In printing machines, the risk of contamination by solid particles is comparatively low and so simple gap seals offer adequate protection. However, these seals do not provide adequate protection from liquids that could be present during machine washdowns. In this case, effective labyrinth seals or contact seals may be more appropriate to the operating conditions.
Dr Steve Lacey is engineering manager at Schaeffler UK
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