Introduction to Maximum Allowable Stem Torque (MAST)
August 13th, 2024
Maximum Allowable Stem Torque (MAST) is a crucial parameter in valve design and selection, particularly when determining the compatibility between a valve and its actuator. MAST represents the highest amount of torque that can be safely applied to the valve stem without causing damage to the valve components. Exceeding this value can lead to stem deformation, valve failure, or even catastrophic damage to the system.
MAST is defined by the valve manufacturer based on the mechanical properties of the valve stem material, the geometry of the stem, and the overall design of the valve. It ensures that the actuator’s output torque does not surpass the valve’s capacity, thereby protecting the valve from excessive stress during operation.
Calculation of MAST
The MAST is calculated using the following formula:
Square Stem：
MAST=(B^3 * τ ）/6
Where:
b is the side length of the square shaft (in meters).
τ is the shear strength of the material (in Pascals, Pa).
6 is a constant derived from the shape factor for square cross-sections.
Round Stem：
MAST=（π×d^3×τ）/16
Where:
d is the diameter of the round shaft (in meters).
τ is the shear strength of the material (in Pascals, Pa).
π is a constant (approximately 3.14159).
Example Calculation For Square Stem:
Let’s consider an example where we need to calculate the MAST for a valve with a square shaft made of 410 stainless steel, which has a shear strength (τ) of 400 MPa (400 x 10^6 Pa). The side length of the square shaft is different for various valve sizes (DN):
DN 50: 9 mm
DN 50 (Square Shaft Side Length = 9 mm = 0.009 meters):
MAST=【（0.009）^3*400* 10^6】/6=48.6 NM
For Round Stem Calculation Process Same to the Square, need turn the mm to the M, and Mpa to Pa.
Conclusion：
By calculating the MAST for each valve size, engineers can ensure that the selected actuator does not exceed the valve’s torque limits. This precaution is essential for maintaining the integrity and reliability of the valve system, preventing potential mechanical failures. When the calculated MAST is lower than the actuator’s output torque, engineers may need to consider increasing the shaft size or selecting a material with higher shear strength to accommodate the operational demands.
