Shaft Tolerances And Hole Tolerances

If you've ever held two parts that should fit, a shaft sliding into a hole. Only to find one's too loose or too tight, you've run into a tolerance problem. 

Tolerances are important in the manufacturing, the tiny allowable size variations that ensure parts work together as designed. 

Today, let's learn two of the most critical: shaft tolerances and hole tolerances.

What  Are Tolerances

 Every part has a nominal size—the ideal dimension on the drawing, like 10mm shaft or 10mm hole. But no manufacturing process is perfect. A lathe might cut a shaft a hair thicker, a drill might make a hole slightly narrower. Tolerances define how much that size can vary without ruining the part's function.

Shaft tolerance: The allowable size range for an external feature, like a rod, pin, or axle that fits into something else.

Hole tolerance: The allowable size range for an internal feature, like a bore, socket, or opening that something fits into.

These aren't random numbers. They're carefully chosen to balance two needs: function because parts must fit and work and cost, tighter tolerances mean pricier machining.

What the Codes Mean

Open a shaft / hole parts drawing, and you'll see drawings marked with codes like Ø10 H7/g6. The Ø10 means 10mm in diameter.

Hole tolerance H7 : The H means this is a hole tolerance band, and 7 is the tolerance grade, a standardized scale from IT01, ultra-tight, to IT18, loose. For a 10mm hole, H7 translates to a specific range: the hole can be as small as 10mm and as large as 10.015mm (numbers vary by size, check ISO standards for specifics).

Shaft tolerance g6: The g here is for the shaft tolerance band, and 6 is its grade. For our 10mm shaft, g6 means it can be as large as 9.987mm and as small as 9.978mm.

The following picture shows more details:

Clearance Fit: When Parts Need to Move

Interference Fit: When Parts Need to Stay Locked

Sometimes you want parts to stick together permanently, like a gear pressed onto a motor shaft. That's an interference fit. Here, the largest possible shaft is bigger than the smallest possible hole. You'll need pressure to assemble them, but once together, they won't slip.

Transition Fit: When You Need a Little of Both

Transition fits land between clearance and interference. Depending on the actual sizes of the shaft and hole, you might get a snug fit or a tiny interference. These are perfect for parts that need to align precisely but still be disassembled if needed.

Shaft and hole tolerances might seem like tiny details, but they're the backbone of manufacturing. Get them right, and your parts fit, function, and stay affordable. Get them wrong, and you're stuck with wobbles, jams, or overpriced parts that don't add value.