Many times hose clamps display leaks even after the application of sufficient installation torque & people are tired of finding out the root cause of this issue & end up with excess tightening of the clamp as a solution. Today in this article I will shed some light on a much unknown & a topic on which we hold our tongue very often, the root of these unknown leaks- the Cold Flow or the Cold Leaks…
1. Cold Leaks -
The hose which is the prime entity of our story is made of elastomer (rubber) which has a bad habit of deformation and is expressed in a term called Compression set (C-set). Compression set is the measurement of how well a material can revert back to its natural shape after being compressed by a load. A material which returns near its original thickness is said to have an ideal compressive strength.
When a hose clamp is tightened on a hose it seeks to reproduce the type of indent (impression) similar to the clamp band which is tightened on the hose. After the introduction of the torque, the clamp is now tightened in one shot & the hose now is not given sufficient time to get into action (rebound) so now with time, the hose compound will slowly move out from under the clamp band to the region of less band force to relax itself. The so called flow of hose can be simulated with a help of a frugal method depicted in fig-1 below wherein a clay is kept under the hose clamp & tightened to installation torque. The clay flows out from the clamp band to the region of no load.
Figure 1- Clay wrapped under the hose clamp before introduction of installation torque Figure 2-An Exaggarated Frugal Simulation of Hose flowing under the clamp band with the help of clay
This relaxation of the hose is perceived as a drop in the installation torque which results in lower band tension force which subsequently leads to leakage & slippage of the joint. This drop in the installation torque can be easily determined using the Relaxation tests. To minimize these relaxation losses, the Dual-step tightening process should be adapted so that the hose can have some time to relax itself before the complete delivery of the torque so that the lost torque can be quickly replaced by the new torque which is being currently applied. Truly speaking, manual tightening of clamps is the best way to fix this issue. If manual torquing is not possible at all joints at least the joints demanding critical applications should be turned manually.
2. The Thermal Cycle-
Another important dimension of this issue is the “Thermal cycle” which is now going to add more fuel to the fire. Initially when the clamp is tightened the system is at an ambient temperature. When the vehicle starts running & reaches its standard operating temperature there is a thermal expansion of the spigot (metal pipe/tubing) & the metallic hose clamp. This expansion of the spigot may be greater than that of the hose clamp due to its direct contact with the heat source. This differential expansion causes an increase in the compressive forces applied to the hose which results in movement of the hose material. Now when the engine is turned off it will slowly return to the ambient temperature. Now at ambient temperature, the thermal contraction of the spigot may be more than the thermal contraction of the Hose clamp. This differential contraction may lead to reduced clamping forces at the joint leading to leakage & slippage of the joint. To counter this issue in the case of thermal cycle, clamping experts have suggested the use of Constant Tension or self-tightening clamps.
Almost all metallic joints expand as a system heats up, and then contract as the system cools down. The conventional worm drive & T-Bolt Clamps are Passive i.e. the expansion and contraction of joints cannot be compensated for without retightening or loosening the clamps. The Constant Tension or the self-tightening clamps are Active clamps that have a mechanism consisting of a spring element that compensates for the temperature changes by actually changing the clamp diameter to cease the cold flow issue completely.
While installation of clamps on vehicle assembly lines it’s a good practice to retighten the clamps after the engine has been warmed up.
To stay safe on the leaking joints periodic retightening of clamps is highly recommended.
Use constant tension/ spring loaded clamps as per your hose & application demand.