Nov28-12, 06:18 AM
I have to assess the sizing of a spindle of a DN600 Knife gate valve.
The spindle is subjected to two sources of loading acting in both shear and tensile.
•Torque applied to the spindle.
•Resulting thrust from line pressure.
Required thrust at 2 bar line pressure: F = 53.2 kN
Corresponding torque: T = 266 Nm
Material: S30400 (BS 970 PART4 GR 304 S15) with the following assumes properties:
Minimum ultimate tensile strength: Sut = 505 MPa
Tensile yield strength: Syt = 205 MPa
Shear ultimate strength: Ssu = 0.6x505 MPa = 303 MPa
Shear yield strength: Ssy = 0.577x205 MPa = 118 MPa
Modulus of elasticity: E =193 GPa
Spindle thread: M40x7 Trapezoidal thread 2 starts; d_m = 36.5 mm
Valve stroke: La = 650 mm
Note: The clearance, fillet radii, surface finish and accuracy of machining have a significant effect on the actual stresses and has not been taken into account in the design calculations.
I have done strength calculations on the thread and the root of the spindle, I also did some calcs to check the spindle for buckling. So here goes, haha...
Stresses acting on threads
Where thread length is
Assuming only 2 threads take the load and there are 2 starts, n will be equal to 4
For TR thread
The safety factor against bending is:
Maximum shear stress
Because bending is involved, the shear stress distribution is not uniform; it occurs at the centre of the thread and is 50% higher than the average stress. At this point the bending stress is zero because maximum bending stress occurs at the outer fibres.
The safety factor against shear is:
Stresses acting on spindle root
Axial compression stress in root
Tensile stress area
Hence compression stress
Polar moment of inertia for spindle root
Resultant torsional shear stress on shaft:
Combined stresses on spindle:
σ_x = -60.6 MPa and Tau_xy=37.7 MPa
From the maximum distortion energy theory and Mohr’s Circle we have
And principal stresses
Shear safety factor:
Tension safety factor:
Compression safety factor:
Because the load in the spindle is compressive when the blade of the valve bottoms out, the spindle is effectively a column and the possibility of buckling should be checked.
We assume that the spindle is flexible (pinned to blade) at one point and rigid at the other point. As per Mechanical Design Manual February 1999, the relationship between effective length Le and actual length La should be Le: 0.85La.
Radius of gyration for circular shafts
This value is less than the limiting slenderness ratio for steel, the Johnson formula applies for the critical buckling load.
F_cr=A_t *S_ty [1-(S_ty*S^2)⁄(4*π^2*E)]
Buckling safety factor:
In this knife gate valve the blade bottoms out on the body casting, so I recommended that if the company are to fit an electric actuator that they make sure the torque settings and sizing of the actuator is set to the correct values to make sure that if the stroke has not been set right that the actuator does not exceed the maximum torque allowed without buckling...
From the calculations the spindle should be adequate to transfer the required torque and resulting thrust with a minimum safety factor of 2.45.
I know it is a lot to check but is someone would please help me check this it would be much apreciated, I am only in my 2nd year of working so would like some experienced input.
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