Alignment Checks

Part of Gear Making

Verifying that gear shafts, bearings, and housings are properly aligned for smooth, long-lasting gear operation.

Why This Matters

A gear pair that is perfectly designed and carefully manufactured can still fail quickly if the shafts carrying the gears are misaligned. When shaft axes are not parallel (for parallel gears) or not at the correct angle (for bevel gears), the tooth load concentrates at one end of the tooth face rather than spreading evenly across it. This edge loading increases contact stress enormously — a shaft misalignment of just 0.5 mm across 100 mm of face width doubles the contact stress at the loaded edge. The result is rapid wear, tooth breakage, and bearing failure.

In a hand-tool manufacturing environment, achieving perfect alignment requires patience and systematic checking rather than precision machinery. The methods are simple but must be applied consistently. Small errors accumulate — a bearing that is 0.3 mm out of position plus a housing that is 0.2 mm misaligned gives a combined misalignment of 0.5 mm that could be the difference between a gear that lasts ten years and one that fails in one.

Understanding what you are checking and why makes the process more reliable. Each check targets a specific alignment error. Doing them in the right sequence catches errors before they are compounded by subsequent assembly steps.

Types of Misalignment

Parallel misalignment (offset): The two shaft axes are parallel but not collinear — one is displaced sideways relative to the other. In a simple spur gear pair, this changes the center distance, which alters the pressure angle, contact ratio, and backlash. Small amounts are tolerable; larger offsets cause heavy contact on one flank and insufficient contact on the other.

Angular misalignment: The shaft axes are not parallel — they converge or diverge in the plane of the gears. In spur gears, this causes helical loading across the tooth face. The tooth touches along a diagonal line rather than across the full face width. This produces very high stress at one corner.

Axial misalignment: Gears are displaced along the shaft axis. Spur gears are relatively insensitive to this; helical gears and bevel gears are more sensitive.

Combined misalignment: In practice, all three types may be present simultaneously.

Checking Bearing Housing Alignment

Before mounting the shafts, verify that the bearing housings (or bearing surfaces in the frame) are correctly positioned relative to each other.

Check 1: Bore centreline height. For a simple two-shaft gear train where both shafts should be horizontal, measure the height of each bearing bore centerline from the reference surface (typically the machine bed or frame base). A steel rule or vernier caliper measuring from the base to the bore edge, plus half the bore diameter, gives the centerline height. The difference between the two centerline heights should be within ±0.1 mm for precision gearing, ±0.25 mm for general industrial gearing.

Check 2: Shaft spacing. Measure the distance between the two shaft centrelines. This must match the design center distance to within the tolerance stated for the gear class. For normal commercial gears, ±0.1 mm on center distance is acceptable. Use a precisely made gap gauge (a known-diameter rod) between the bearing bores rather than a ruler for better accuracy.

Check 3: Parallelism. Place a precision straight bar or straightedge in each bearing bore and measure the gap between them at each end of the shaft length. If the gap is the same at both ends, the bores are parallel. A difference indicates angular misalignment.

Shaft Mounting and Checking

Once shafts are mounted in their bearings, re-check alignment with the shafts in place — bearings are not always concentric with their housings.

Dial indicator check. A dial indicator (or improvised equivalent using a bent wire and scribed mark) clamped to one shaft and resting against the other shaft detects runout and misalignment as both shafts are rotated. For parallel shafts, read the indicator at four positions (top, bottom, both sides). If the reading changes between positions, the shafts are either bent or misaligned.

Straight edge across gear faces. For unmounted gears, hold a straightedge across the face of both gear blanks simultaneously. The straightedge should contact both faces evenly — any gap at one end indicates angular misalignment of that amount across the gear face width.

Gear Mesh Checking

After mounting gears, check the mesh directly:

Backlash measurement. Hold one gear fixed and rock the other back and forth. The angular movement before the teeth engage is the backlash. Measure it at the pitch circle (the mid-tooth height point) using a lever or feeler gauge. Target backlash depends on gear size and application — for a gear with 100 mm pitch radius, 0.2–0.5 mm linear backlash at the pitch circle is typical.

Contact pattern check (Prussian blue test). Apply a thin smear of marking compound (Prussian blue, engineer’s blue, or improvised alternatives like soot rubbed in oil) to the teeth of one gear. Mesh the gears lightly under hand pressure and rotate through several revolutions. Examine the pattern left on the unmarked gear’s teeth. A good pattern is a continuous mark across the full face width, centered vertically on the tooth. Edge marks, corner marks, or broken patterns indicate misalignment and must be corrected before the gear is put into service under load.

Correction Methods

If alignment checks reveal problems:

• Center distance error: Adjust bearing position by shimming (adding thin metal shims under the bearing housing) or by re-boring the housing at the corrected location.
• Height mismatch: Shim under the lower bearing housing.
• Angular error: Adjust one bearing housing angularly by shimming one side.
• Combined errors: Work systematically — fix the largest error first, recheck, then address remaining errors.

Shimming is the preferred correction method because it is reversible and adjustable without machining. Cut shims from thin sheet metal (brass is easy to work) to the precise thickness needed. Stack multiple shims for fine adjustment.