High Aspect Ratio Threadform for Gate Valve Stem
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Solution Overview
Problem
Existing threadforms for gate valve stems under high pressure and large bore conditions experience non-uniform contact pressure, leading to inefficiency and galling due to fixed thread profiles that do not adapt with increasing diameter, resulting in excessive bearing stress and failure of surface treatments.
Innovation Solution
A high aspect ratio thread form with a thread height to thread pitch ratio of at least 1.1, featuring triangular configuration with rounded crests and roots, and an included angle less than 50 degrees, designed to flex and distribute load uniformly, reducing peak bearing stress and friction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the thread profile height is kept fixed in the prior art, then the manufacturing is simpler, but the thread bearing stress increases more than double when going from 0.5" to 2.0" stem diameter
Solution Approach 1:
The patent changes the thread profile parameters by introducing a variable height profile where the thread height increases with stem diameter according to the formula h = 0.5 * d^0.6. This parameter change allows the thread bearing area to increase proportionally with diameter, keeping the bearing stress relatively constant across different valve sizes rather than allowing it to more than double.
2Area of stationary object
If the thread engagement length is made longer in the prior art, then the thread bearing area should increase, but the threads along the length do not load up evenly so additional threads provide essentially no benefit
Solution Approach 1:
The patent applies local quality by concentrating the thread loading in the upper portion of the engagement length. The variable height profile ensures that the majority of the load is carried by threads near the top where the stress is highest, while threads lower down carry progressively less load. This creates an optimized local distribution of load across the engagement length rather than attempting to utilize the full length uniformly.
3Strength
If the stem diameter is made larger to withstand higher pressures, then the tensile strength increases, but the thread bearing stress increases more than double because the thread bearing area does not increase at the same rate
Solution Approach 1:
The patent changes the thread geometry parameters to match the scaling of the stem diameter. By using the formula h = 0.5 * d^0.6, the thread height scales with diameter in a way that maintains a relatively constant bearing stress. This allows larger stems to withstand higher pressures without proportionally increasing the thread bearing stress, resolving the contradiction between needing larger diameter for strength and avoiding excessive bearing stress.
4Reliability
If the modified ACME thread is hardened and coated with dry lubricant, then the surface treatment provides initial protection, but under high pressure the non-uniform contact pressure overloads the surface treatments resulting in galling
Solution Approach 1:
The patent changes the thread profile parameters to create a more uniform contact pressure distribution. The variable height profile h = 0.5 * d^0.6 ensures that the contact pressure is distributed more evenly across the thread flanks, preventing any single area from being overloaded. This reduces the stress on surface treatments and lubricants, preventing galling and friction failures even under high pressure conditions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The high aspect ratio thread form significantly reduces peak bearing stress by up to 5 times, minimizing galling and friction, and achieving thread efficiency improvements of up to 50% by distributing load more evenly and increasing contact area.
Implementation Method 1
The thread stiffness is designed so as to provide flexibility and greater contact area. By maintaining a high thread aspect ratio to the above relationship, the threads will flex so that flank to flank contact emanates from the midpoint of the flanks
Data Source
AI summary
A threadform for a gate valve has a set of threads on a stem that engage a threaded receptacle of the gate. When the stem is rotated, the gate moves. The threads have a thread height to thread base thickness ratio that is at least the inverse of 0.7 times e raised to 0.388 divided by a nominal thread diameter of the threads. That ratio is at least 1.1.