1.615.451.9080 | 430 CALVERT DRIVE | GALLATIN TN 37066 US
March 30, 2019

Gear Worm Wheel

Our redesigned M171 gear worm wheel won the 2008 Investment Casting Institute Military Equipment Award, and is a testament to the benefits of investment casting. The primary function of the gear worm wheel is to guide the movement of the M171 Telescope Quadrant mount for leveling and elevation error compensation.

Originally developed in the 1970s, the M171 Gear Worm Wheel was originally cast using 304 stainless steel, and was designed for use with the 15,772 lb M198, a 155mm tow Howitzer. By applying the unlimited alloy choices offered by investment castings, Precision Castings of Tennessee was able to modify the M171 for use with the lightweight M777 LW155 (7000 lb) without compromising dimensional stability.

Unique Design Considerations:

The M171 design originally used on the M198 proved to be an engineering challenge when fitted to the 42% lighter M777. The increased forces from the recoil of the weapon, which were no longer being absorbed by the massive weight of the weapon system, required an immediate engineering change in order to meet the government fielding requirements of the new weapons system.

PCT engineers achieved these requirements by extending a rib and converting the casting to IC 17-4 PH. Cryogenic heat treatment was then applied to provide dimensional stability, lower residual stress, improve toughness, and maximize service life.

Primary Customer Benefits:

PCT improved the customer's manufacturing process by applying current casting and metallurgical technology to a legacy product. PCT was able to cast the M171’s complex geometry required in a net shape part, reducing the overall manufacturing cost.

Investment casting’s alloy versatility made it possible to meet manufacturing requirements without the need for expensive, time consuming tooling changes and/or process changes. The customer was also able to continue using the existing machine tooling, CNC programming, and inspection fixtures, while retaining interchangeability with M198, M777, and other artillery.

Special design requirements and/or process requirements:

The gear worm wheel was originally designed with certain physical characteristics and property requirement for a specific weapon system. The options were to either redesign the geometry of the part to withstand the additional load, or change the material to increase the physical properties of the casting to satisfy the new load requirements.

Geometry changes could have added weight that would have interfered with the accuracy of the weapon or eliminated the interchangeability of the M171 mount. By opting to change the alloy and apply cryogenic heat treating, PCT was able to remove residual stresses without changing dimensional stability.

Material used (composition of metal of alloy):

IC 17-4 PH (AMS 5355)

Size of part:

The weight of the casting is approximately 7.5 lbs with outside footprint dimensions of approximately 9”x 9”x 6”.

Secondary operations performed:

Precision straightening, heat treating, cryogenic treatment, non-destructive testing, and machining