Hard drive to accuracy

Bearings with molecular accuracies designed to maximise the capacity of hard disk drives are pushing the frontiers of the technology, writes Mark Venables

With the unremitting growth of the personal computer market, minimising the size of hard disk drives while maximising their capacity has been a primary focus of computer manufacturers. Bearings have played a crucial role by ensuring that this new breed of smaller, faster hard disks performs with the required precision when transferring information.

The unique requirements of this function have led to the development of specialised bearing designs, greases, seals and cages, while the performance of bearings for computers is now accurate enough to be described in nanometres – one thousandth of a micron.

More sophisticated software and the greater capacity needed to process audio and video data has led to hard-disk drives with gigabytes of memory, compared to several megabytes a few years ago. The trend is set to continue: the memory capacity of 3.5-inch magnetic disks, now about 2-3GB, is expected to increase to about 5-7GB by the end of the year. Partly this will be due to increased memory density of the recording surface and improved magnetic heads. But better accuracy in the bearings will again play an important role.

The measurement for bearing accuracy is NRRO, or non-repetitive run-out. This technique is used in processes where synchronisation of movement is important, says Tony Cuff, engineering manager at bearing supplier NSK-RHP in Nottingham.

`The transfer of information to and from computer disks is one such example,’ he adds. `In information transfer, any vibration that is not synchronous with the characteristic rotational speed of the hard disk spindle drive has a detrimental effect on quality or system performance.’

Improving accuracy

`In NSK’s method for measuring NRRO, a single ball bearing is mounted with an air bearing and driven by a direct-drive motor,’ says Cuff. `Using this method, the NRRO of a typical 5mm bearing for HDD spindles comes out at around 0.05 microns. This is nearly half the level of NRRO only four or five years ago.

`Considering that the molecular size of tobacco smoke is in the order of 0.1 microns, the accuracy of this bearing is truly remarkable.’

That said, NSK wants to get NRRO even lower. The company is working on improving outer and inner raceway accuracy at the nanometer level, increasing ball accuracy and developing optimal cage designs.

In addition to the demands for higher memory densities and smoother information retrieval, hard disk drives, or their spindle motors, are having to run faster, (up to 10,000rpm) to achieve faster data transfer. To make this happen, bearing producers are developing rolling bearings with rotating outer rings. But, says Cuff, these have provided demands of their own on bearing designers.

Previously, HDD spindles employed bearings with inner rotating rings. The normal method of sealing these bearings was via metal shields, which provided sufficient sealing at low cost.

However, with the new design of outer-ring-rotating bearing, higher speeds exposed a problem. The fit of the metal shield in the groove of the outer ring is not sufficient to prevent grease leakage.

Because of this, there has been a switch to using rubber seals, which are more flexible and resilient, and which ensure adequate sealing and minimisation of grease leakage.

Tackling emissions

This has brought other benefits because the rubber seals, if made of an appropriate type of rubber, help overcome particle emission problems.

While conventional narrow-width bearings for spindle motors have a seal designed for one side only, this is not acceptable on hard disk drive bearings, whose outer rings rotate at high speeds.

Since the ball and cage rotational speeds in a bearing whose outer ring is rotated are higher than that of one with inner-ring rotation, the centrifugal force acting on the grease is higher, especially when motor speed is high. With this higher centrifugal force, not only the grease base oil but the grease itself may leak from between the rings.

As hard disk drive spindle motor bearings are required to perform with very high reliability, the gradual loss of grease can be a disastrous problem.

For this reason, bearings with an outer ring which rotates at high speed require rubber seals that are capable of two-way sealing.

Plastic cages are increasingly indispensable for the type of bearings used in hard disk drives. These type of cages are more suited to modern engineered structures as they weigh less and so can be used at high rotational speeds. They also make less noise when in use, which is a benefit in equipment such as computers and videorecorders.

Compared to ordinary grease-lubricated bearings, where grease occupies 20-30% of a bearing’s internal space, the grease content in such a spindle motor bearings is only about 10-20%. While this low-grease content is necessary to meet requirements for low torque and NRRO, it is not conducive to good lubrication. However, the self-lubricating property of plastic cages helps to overcome this.

It was estimated that 160 million units were made in 1999, based on the higher-than-10% growth rate in hard disk drive production in the past few years. Demand is expected to grow further this year and even more intensive technical demands are sure to follow.