Question 1

How do I choose between pressing and thermal assembly?

Accepted Answer

Press assembly (hydraulic press) is simpler but limited by friction during insertion — it can gall or seize with large interference. Thermal assembly (heating hub to 150-300°C or cooling shaft with liquid nitrogen at -196°C) allows slip-fit assembly with zero friction, achieving the full design interference. Use thermal assembly for interference > 0.001×diameter, long contact lengths (L/d > 1.5), or when surface finish must be preserved.

Question 2

What surface finish is required for press fits?

Accepted Answer

Both shaft and bore surfaces should be Ra 0.8-1.6 μm (ground or hard-turned). Rougher surfaces reduce effective interference because peaks deform plastically, while very smooth surfaces (<Ra 0.4 μm) can reduce friction coefficient and thus holding capacity. Surface finish should be specified as cylindrical grinding with no spiral pattern, which can create a helical leak path.