Measuring the hardness of a gear

Brief description of a gear

Gear is the main element of the transmission mechanism. It is a disc with teeth projecting radially. Two wheels form a gearing pair to transmit rotational or translational motion.

Metal gears differ in the type and shape of the tooth profile, surface shape, material, and size. The market offers a wide variety of gears: spur and helical; worm gears, and wheels with internal gearing; sprockets, and racks; bevel, hypoid and non-circular gears.

They are part of vehicle mechanisms, construction equipment, pumps, industrial equipment and even watches. Metal gear wheels are made of steel (including carbon and alloy steel), less often of cast iron.

Hardening of gear wheels

To form a hard diffusion layer on the surface of the gear wheel (with more than 350 HB), different types of thermal, and thermo-chemical hardening are used. The most common methods for hardening steel teeth are:

• thermo-chemical treatment (carburizing, nitrocarburizing, and nitriding).
• surface hardening with HEH HFC (high energy heating by frequency currents). It is used for steels with a carbon content of 0.35 - 0.6%. Not suitable for hardening wheels with fine teeth (less than 5 mm), because excessive hardness increases their fragility.
• HEH HFC method is perfect for the treatment of small teeth(less than 3 mm).

Various parts of gear can be treated with HEH HFC:

• the surface of a tooth (A);
• the surface and the whole body of the tooth (B);
• top land and face (C);
• bottom land (D);
• whole tooth and bottom land (E);
• along the contour with different depths (F);
• along the contour with the same depth (G);
• the surface of the tooth and bottom land (I).

Heat-treated gears have increased transmission load capacity, better wear properties, and galling resistance, and increased bending endurance. That is why careful selection of hardness measurement equipment and subsequent hardness control of steel teeth is required.

Features of the hardness control of gears

To measure the hardness of the surface layer of gear with a hardening thickness of 30 to 100 mcm and roughness of up to 3,2 Ra, a portable ultrasonic hardness tester of the TKM-459СЕ series with an additional "H" type probe is suitable. Small weight load is one of its main advantages, allowing dealing with thin surface layers and small-sized products.

Probe positioning requirements

Depending on the hardening method (see picture), the measurement can be carried out in different areas.

• When inspecting the top land it is important to position the probe in accordance with the surface geometry.
• When inspecting the bottom land, geometry should also be considered. If the spot is hard to access, you can remove the protective cap from the probe. In this case, the rod lateral surfaces must not come in contact with anything!
• When inspecting the surface, it is required to monitor the perpendicularity of the spout.

Important! When the workpiece is a small gear wheel weighing 0.1-1 kg, fix it to the surface using a vise or fixing paste.

To avoid wrong choices, do not hesitate to send us the drawing of your workpiece and let us select the best equipment to match your tasks.