Click on the advert above to visit the company web site

Product category: Lubricants
News Release from: NSK UK | Subject: ECA grease
Edited by the Engineeringtalk Editorial Team on 29 August 2000

The importance of grease in Xerography
mechanics

Correct grounding is fundamental to Xerography, and to permit electrical discharge, NSK have developed bearings with an electrically conductive grease and nitrile rubber contact seals

Xerography in both printers and copiers consists of seven interrelated processes, Charging, Imaging, Exposing, Developing, Transferring, Fusing and Cleaning, all of which occur simultaneously and centre around a piece of equipment called the photoreceptor Correct grounding is fundamental to the operation of photoreceptors and magnetic rolls used in the xerographic process

If discharging could not take place these components would malfunction or cause print quality to suffer.

However, in many cases the electrical passage to ground has to take place through the photoreceptor and magnetic roll bearings.

To permit electrical discharge, NSK have developed bearings with an electrically conductive grease and nitrile rubber contact seals.

The ECA grease consists of special heat resistant carbon-black, and a synthetic oil base which provides good oxidation stability, heat resistance and lubricity.

At 25C the resistivity of ECA grease, at 2000 Ohm.cm, is 300 times less than other greases containing carbon-black and 350 million times less than lithium soap greases.

In addition, compared to conventional greases where conductive particles are gradually dispersed from the rolling contact surfaces with bearing operation, the resistance change with time is small for ECA grease.

This is because the electrically conductive carbon-black particles are very small in comparison with the particles in conventional greases.

Apart from the problem of ensuring adequate grounding for photoreceptors, care must also be exercised in the choice of associated components because of the possibility of damage resulting from chemical and oil contamination.

In respect of bearings, grease leakage can be kept to a minimum by employing NSK bearings with standard contact seals.

In torque critical applications, too, NSK's light contact DDW seals have demonstrated excellent grease retention characteristics.

Added to this, both types of seals are also very effective against the ingress of toner contamination.

In another of the xerographic processes, hot roll fusing, several features influence fuser roll bearing design.

Some of these are the type of toner and the print rate, and whether the bearing is fitted to an insulating sleeve or not.

All of these affect the temperature 'seen' at the inner ring of the bearing.

Some toners require a higher fusing temperature than others.

A high print rate means that there is less time available for the fusing process to occur, and this feature in itself means that a higher fusing temperature is necessary.

An insulating sleeve in the bore of the bearing can reduce the bearing inner ring temperature by as much as 50C.

A recent trend in fuser roll design has been to incorporate a larger heat lamp or additional lamps.

Under these circumstances it is necessary to increase the diameter of the fuser roll tube, and in order to retain a compact bearing design, the insulating sleeve is omitted.

Typical inner ring temperatures for non insulated bearings can range from 160C to 200C depending on the design of the fuser roll assembly, whereas outer ring temperatures can be as low as 120C.

These inner ring temperatures mean that both insulated and non insulated fuser roll bearings are generally of a very high specification.

Typical bearings incorporate either polyacrylic (temp to 150C) or viton (fluorine rubber) contact seals (temp to 200C), ISO C5 radial internal clearance, dimensionally stabilised rings, and a special grease with a high temperature capability and long grease life.

An additional problem at operating temperatures above 120C is that the crystal structure of bearing rings begin to change and retained austenite decomposition gradually increases.

Under these conditions bearing rings can undergo diametrical expansion, leading to creep on the fuser roll shaft and risk of radial preload due to loss of radial internal clearance.

To minimise this problem NSK recommend X26 or X28 dimensional stabilisation which are specifically aimed at operating temperatures of 150C and 200C respectively.

At these temperatures grease specification is also critical.

NSK recommend either ENS for operating temperatures up to 160C, or KP1 grease for temperatures up to (and sometimes exceeding) 200C.

ENS is a standard grease within the NSK organisation and was developed by NSK for high temperature conditions.

It has a synthetic ester base oil for good oxidation stability, heat resistance and low torque characteristics, and a urea compound thickener for optimum shearing stability.

ENS has a long grease life even at 160C, which is particularly advantageous in conditions of toner contamination.

It is also a quiet running, NSK noise 'class A' grease, with low leakage characteristics.

NSK's standard grease for fuser roll bearing temperatures in excess of 160 C is KP1.

This grease has a PTFE thickener and a fluorine base oil for high temperature resistance.

Grease lives of approximately 3000 hours are achievable at 200C with KP1, even in conditions of toner contamination, and compared with usual commercial fluorine base grease, it has a significantly better seizure life.

These characteristics are of particular advantage when printer manufacturers wish to extend the life of a fuser roll assembly, and maximise print quantity.

Although the pressure roll is in contact with the fuser roll, pressure roll bearings run significantly cooler than fuser roll bearings, and are typically shielded.

NSK's 'ENS' grease is recommended for this application (see above).

A number of components within printers and copiers require cooling.

Exit blowers or motor driven fans are used for this purpose and commonly require miniature ball bearings.

All NSK bearings are 100% noise tested with the majority automatically meeting stringent electric motor ('E class') noise quality standards.

However for exit blower motors, NSK can guarantee electric motor noise quality.

The result of specifying 'E class' bearings is an extremely quiet fan motor.

ENS grease is recommended for motor applications as it has a high speed capability and low leakage characteristics.

Its shearing stability is excellent and it does not soften or liquefy even at high speeds.

THE XEROGRAPHIC PROCESS EXPLAINED...

Laser printers commonly operate with negative charging and one component developing.

When computer data is sent to the printer the fusing system heats up to the specified temperature, and the photoreceptor scorotron is activated with a high voltage negative dc power supply.

This voltage induces a charge on the photoreceptor.

The scorotron assembly consists of a fine platinum or tungsten charging wire, insulated shield and wire grid.

The scorotron wire is very fine and is connected to a grounded power supply.

The shield surrounds the wire on three sides, and the remaining 'open' side, adjacent to the photoreceptor, is covered by the wire grid.

As with the scorotron wire, the scorotron grid is connected to a grounded power supply.

When a voltage is induced across the scorotron wire, electrostatic fields are developed between the wire and shield, the wire and grid, and the wire and photoreceptor.

These fields free electrons from the air molecules immediately surrounding the wire, and repels them outwards.

As these electrons move they collide with adjacent air molecules freeing further electrons, and the surrounding air molecules are positively charged or ionised.

This process continues until the air surrounding the scorotron wire is saturated with positive ions and free electrons (or negative ions).

Some of these free electrons move towards the scorotron shield but are repelled due to fields between themselves and the electrons on the inside surface of the shield.

Electrons are now repelled from both the wire and the shield and pass through the scorotron grid towards the photoreceptor.

The function of the scorotron grid is to control both the strength and uniformity of the resulting negative charge on the surface of the photoreceptor.

At this stage the photoreceptor is rotating, and the scorotron deposits a uniform negative charge over its surface.

As the surface of the photoreceptor is manufactured from photochromic material, and the charging process occurs in the dark, the surface layer of the photoreceptor acts as an insulator.

To maintain the negative charge at the surface of the photoreceptor, the substrate layer (below the surface) of the photoreceptor is grounded, and electrons in the substrate are conducted to ground.

This creates a further electrostatic field between the surface of the photoreceptor and the substrate, which holds the negative charge on the surface of the photoreceptor.

At the same time as the scorotron is activated, a high voltage positive dc power supply activates the transfer corotron, which is used to charge the paper prior to fusing.

The transfer corotron is similar to the scorotron, however no grid is needed, the corotron shield is grounded and the resulting charge on the paper is positive.

After the fusing system has heated up and the charging processes are complete, the computer data is converted into on / off switching of a laser.

The beam is reflected off a series of 'first surface' mirrors and then 'fired' through a spinning polygon.

The polygon ensures that the light beam is swept across the photoreceptor surface, so that the charge is reduced to correspond with the shade or density of the desired image, or almost completely discharged to correspond with the non text areas of the page.

The photoreceptor acts as a conductor in the regions exposed to light and the charge is conducted to ground through the substrate layer.

Lenses ensure that the beam shape is circular and that the beam scan is across the photoreceptor.

The result of this process is to form a 'latent' image on the photoreceptor.

A developer roll (or magnetic roll), positioned adjacent and parallel to the photoreceptor, rotates and attracts toner to its surface.

The developer roll consists of a hollow tube containing fixed magnets.

As the tube rotates, the magnets cause the toner (which is typically iron embedded in a plastic resin) to adhere to the roll.

Further rotation of the magnetic roll carries the toner past a metering blade and the quantity of toner carried to the photoreceptor is regulated.

The toner is then moved to the photoreceptor by applying a dc charge of the same polarity to the magnetic roll, and the toner is literally repelled to the rotating latent image on the photoreceptor.

Paper moves from the paper tray and is charged by the transfer corotron.

The transfer corotron generates a positive charge on the paper, which attracts the negatively charged toner, and allows the paper to adhere to the photoreceptor.

The paper is then 'stripped' from the photoreceptor.

As the paper is stripped from the photoreceptor it has toner statically attracted to its surface and resembles the text on the computer screen.

After leaving the photoreceptor, the paper passes through a fuser and pressure roll assembly.

The fuser roll (which is sometimes called a heat roll) contains a heat lamp, and is positioned above the pressure roll.

As both rolls are parallel and contact one another, the fusing process melts the toner into the paper fibres under both heat and pressure.

The finished copy is then stripped from the fuser assembly and transferred to the output tray.

Any remaining charges within the copier are neutralised, and residual toner is removed from the photoreceptor with a cleaning blade. Request a free brochure from NSK UK ...

• NSK UK: contact details and other news
• Email this article to a colleague
• Register for the free Engineeringtalk email newsletter
• Engineeringtalk Home Page

Search the Pro-Talk network of sites

Put your news on Engineeringtalk  |   Advertise  |   Get the free Engineeringtalk newsletter  |   Engineeringtalk Home
About Engineeringtalk  |   Copyright © 2000-2008 Pro-Talk Ltd, UK