Piston Retention via Thread Root Radius and Shoulder Load Bearing
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Solution Overview
Problem
Prior piston and rod assemblies face issues with thread fatigue and loosening under high torque applications, leading to increased production costs and stress riser risks, as they rely heavily on threaded engagement for retention, which is inefficient and prone to failure.
Innovation Solution
The proposed solution involves a piston and rod assembly design with a threaded segment and root radius greater than 0.125 pitch, allowing for low torque engagement (up to 1000 Nm) and the use of a retaining member with setscrews to prevent loosening, reducing reliance on high torque and distributing axial loads effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high torque is applied to tighten the nut to the rod end for retention, then the threaded joint can withstand higher loads, but the threads become highly susceptible to fatigue and loosening, and the rod end requires higher strength materials and heat treatment increasing costs
Solution Approach 1:
The rod end is segmented into two functional zones: a reduced-diameter threaded portion for nut engagement and a full-diameter shoulder portion for load bearing. This segmentation allows the threaded portion to be optimized for thread engagement while the shoulder portion handles the majority of axial loads, reducing stress on the threads and eliminating fatigue susceptibility.
Solution Approach 2:
The load-bearing function is extracted from the threaded joint and transferred to the rod shoulder. By designing the rod with a shoulder that contacts the piston, the patent removes the threads from the high-stress path, allowing them to engage at lower torques without fatigue risk while the shoulder handles the full axial load.
2Ease of manufacture
If the rod end is machined to reduced effective cross-sectional area for nut engagement, then the nut can be coupled to the rod end, but the smaller cross-sectional area must be processed with higher strength materials and heat treatment, increasing production costs
Solution Approach 1:
The rod end is divided into a reduced-diameter threaded section and a full-diameter shoulder section. The reduced section is only machined to the extent needed for thread engagement, while the shoulder retains full diameter for load bearing. This minimizes the amount of material removal and eliminates the need for heat treatment of the entire rod end.
Solution Approach 2:
The rod end has different diameters in different regions: a smaller diameter where threads are needed and a larger diameter where load bearing is needed. This local variation in geometry allows standard material processing while achieving both nut coupling and load bearing functions without requiring higher strength materials or heat treatment.
3Ease of manufacture
If standard M-class threads with small root radius are used for piston and rod engagement, then the threaded connection can be formed, but the stress concentration at the root increases susceptibility to fatigue and failure under heavy duty applications
Solution Approach 1:
The thread root radius is increased from the standard M-class value of 0.125P to a larger value of 0.25P to 0.5P. This parameter change reduces stress concentration at the thread root, significantly improving fatigue resistance while still allowing standard threading processes to be used for manufacturing.
4Strength
If only the first three threads between the nut and rod end are used to withstand load, then the threaded joint can be formed with high torque, but the remaining threads are underutilized and the joint is prone to loosening under axial loading
Solution Approach 1:
The load-bearing function is extracted from the threaded joint and assigned to the rod shoulder. This allows the threads to engage at lower torques with all threads equally loaded, while the shoulder handles the majority of axial loads, preventing loosening and utilizing the full length of the threaded portion.
Data Source
AI summary
An apparatus and method for coupling a piston to a rod to form a cylinder assembly are provided. The piston has a tapped bore extending therethrough for coupling to a first threaded region of the rod. The rod end extends beyond the piston member and includes a second threaded region for coupling to a tapped bore of a retaining member. The piston member is coupled to the rod member at a low torque, e.g., up to about 1000 Nm. An external thread of each of the threaded segment of the piston bore and the first threaded region of the first portion of the rod member may be formed with a root radius of greater than 0.125 pitch or with MJ class thread. The retaining member may include setscrews offset from one another for frictional engagement with the rod end. The thread pitch between the rod and the piston may be coarser than between the rod and the retaining member. An interface and a seal region between the rod and the piston may be disposed closer to the rod end side of the piston member than the threaded coupling.