Hard Drives, Head Readers, Etc.

BTI engineers and manufactures a complete line of manual, semi-automatic, and fully automatic precision measurement and testing equipment for hard drives, head readers, and related components

Data storage rotors are the extreme case for measurement resolution. They are small, they run fast, and the permissible unbalance that follows from those two facts is smaller than the contribution of almost any ordinary fixture. At that point the limiting factor stops being the part and becomes the machine measuring it.

Balance Technology Inc. builds precision balancing and measurement equipment for data storage components — spindle motors, platter and disc stacks, and related high-precision rotating assemblies.

Why the Numbers Get Difficult

Permissible unbalance scales with rotor mass and inversely with speed. A light rotor turning quickly therefore lands at a residual measured in fractions of a gram-millimeter, well below what a general-purpose industrial balancer is designed to resolve.

Two things follow. The measuring system needs a noise floor and a repeatability an order of magnitude better than the tolerance it is checking. And the fixture contribution has to be characterized rather than assumed, because at these levels the tooling can be the largest single term in the measurement.

Air-bearing supports and drives are common in this class of equipment for exactly that reason: a rolling-element support introduces more vibration than the part is allowed to have.

Correction Without Debris

These assemblies are built in clean environments, and the usual correction methods are unavailable. Drilling and milling generate particulate that cannot be tolerated near a head-media interface, so correction moves to methods that produce no swarf — laser ablation, precisely placed adhesive mass, or designed balance features built into the hub.

Every one of those has a repeatability specification of its own, and on a tolerance this tight the correction process contributes meaningful error rather than simply removing it.

Rotational Balance Is Not the Only Dynamic Problem

Head and actuator assemblies do not rotate continuously, so they do not have a balance condition in the conventional sense. What they do have is a dynamic response — resonance, settling behavior, off-track motion under acceleration — which is a different measurement requiring different equipment.

It is worth separating the two when specifying a line. Balancing equipment characterizes the spindle side. Dynamic and resonance measurement characterizes the actuator side.

Setting the Tolerance

Where a balance specification applies it follows ISO 21940-11 from a balance quality grade, the rotor mass and the operating speed, though this class of hardware frequently sits at or beyond the fine end of the standard’s range and may be governed by a customer specification instead. BTI publishes a free ISO 21940-11 balance tolerance calculator with the method behind it — grade selection, the permissible unbalance formula, plane allocation, and a worked example.

What Else Gets Measured

Work in this area commonly combines balancing with:

• runout and axial motion measurement
• resonant frequency measurement
• dimensional gauging of hub and bore features
• functional testing

Measurement system capability studies matter more here than in almost any other application, because the tolerance and the measurement uncertainty are close together.

Equipment Configurations

Layouts are normally engineered to the specific component and its cleanliness requirements rather than selected from a catalog. For platform selection across the full range, see BTI balancing machines, and for the measurement side see resonant frequency measurement systems.

Related applications include armatures, rotors and motor assemblies and spindle and tool holder balancing.

Frequently asked questions

Why do hard drive components need balancing?
Because the disc stack turns at high speed with read heads flying microns above the surface. Unbalance produces runout that the head positioning system has to compensate for, and beyond a certain level it cannot. The tolerances are among the tightest in any balancing application.

How are disc stacks balanced?
As assemblies, on their own spindle, at or near operating speed. Correction is made in very small increments, typically by adding or removing tiny masses at defined locations on the clamp or hub. The quantities involved are small enough that handling and cleanliness become part of the process.

What makes this application different from conventional balancing?
Scale and environment. The residual unbalance permitted is orders of magnitude smaller than on an automotive part, the correction increments are correspondingly tiny, and the whole operation has to happen without introducing particulate contamination. The physics is the same; the practice is not.

Can a disc stack be balanced after assembly into the drive?
No. Once the stack is enclosed there is no access for correction. Balance is established during assembly of the stack, which is why the balancing operation is integrated into the assembly process rather than performed afterward.

What else is measured on these assemblies?
Runout, axial and radial, and the dynamic response of the stack at speed. Because the head-to-disc spacing is the critical dimension, anything that moves the disc surface out of plane matters as much as mass unbalance does.