Failed Spindle Motor Bearings           


Overview
                         
The use of hard drives is not limited to computers these days. Its utility is expanded to several digital devices like cameras, printers, photo copiers, digital video recorders etc. Memory storage in these devices is carried out on the concentric magnetic medium. Spindle motor performs the task of rotation of the disc. The data is stored in the form of magnetic transitions. As the spindle rotates the disc, transducers (heads) attached to the system reads/writes the information.

Spindle motor is positioned below the discs or inside the hub. One prominent type of motor is “in-spindle”, in which spindle is mounted by means of a pair bearings placed just above and below the spindle. These bearings facilitate a speedy rotation of the disc. They also keep the spindle aligned with the shaft. This ball bearing system has plethora of shortcomings attached with it. The most well-known drawbacks are:

* “Non Repeatable Run-Out” (NRRO) – Track Mis-Registration (TMR) is primarily attributed to NRRO, which affects the performance of the system inadequately. NRRO also forbids the system in achieving higher track densities. Due to incessant contact between the ball bearing and the disc, production of NRRO is large.

* “Higher Acoustical Noise”- Due to lengthened mechanical contact between the bearings, a noise of high intensity is created which often is treated as a bad omen regarding the performance of the system.


* “Low Reliability”- Ball bearing is considered less a reliable because of various losses accompanied with it. The efficiency o ball bearing spindle is very low.

Fluid Dynamic Bearing System

The above discussed drawbacks of ball bearing led to the invention of a more reliable and efficient system known as “Fluid Dynamic Bearing” (FDB) system. This new system is based on the principle of hydrodynamics. Fluid dynamic system contains oil in between the stator and sleeve. The viscosity of oil inhibits the production of NRRO. Apart from this achievement there are several other advantages of FDB. Few of these are- ‘improved acoustics’, ‘higher damping’, ‘abridged resonance frequency’, ‘enhanced non operational shock resistance’ ‘better speed control’ etc.



In FDB system, gas or any lubricating liquid serves as media which creates pressure between rotating spindle and stationary base. For a meticulous dynamic performance the gap so created should be controlled effectively. “Precision Machining” is done to obtain a uniform and constant gap between the static base and rotating spindle. The variable gap may lead to NRRO and other performance setbacks. 

Following suggestions may hold good for attaining an effective functioning of spindle motor bearings:

* “Aspect Ratio” (depth to width ratio) should be optimized to obtain a constant gap between the interface of the hub and spindle.

* Materials which show lesser tendency to wear down and remain shock resistant during the operation of the motor must be selected, to tightly hold the gap between them.

* If the spindle motor stops working, then the complicacy should be handled by highly skilled professionals.
                




















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