Military vehicles play a vital role in protecting our society — and inevitably encounter many extreme conditions across their lifespan. From the varying weight, to the rough terrain, there is a lot that can go wrong. Military steering systems must therefore be engineered to take on such extremes. This begins at the design stage and ends with analysis through testing. It's here the parts are tested to their limits and validated against the original specification and real-life requirements.
Testing of steering systems is primarily about replicating as near to the same conditions that would be encountered on the road. Theoretical testing using calculations is a good start, but nothing gets closer to reality than physical testing. Using an on-site test facility with purpose-built test rigs, it is possible to test a subassembly of the entire steering system, presenting the whole structure with like for like conditions that match the final application.
One of the most important parameters to test is the maximum load. With this information we can observe how much force a part can endure, in both tension and compression, before a failure occurs. At Pailton Engineering, there are different rigs to test a range of force applications, our highest being ±400 kN, which is capable of applying static and dynamic loads.
Applying a large force a few times isn't representative of the actual conditions a vehicle will encounter during its entire lifetime. Working loads seen in harsh environments can be just as detrimental to a part’s life as the peak loads. We therefore incorporate the two in order to see how different forces affect the part dynamically.
With enough information from customers, we are able to compile a multitude of loads at their respective frequencies and cycles as part of a block program. This program effectively mirrors the real life data that is gathered from the vehicle. This allows us to accurately assess the true fatigue life of the part.
With a variety of loads and frequencies in place, it is also important to measure the number of cycles that the parts can endure over time. A typical test at Pailton can perform 1,000,000 cycles in only 1 week. That's enough to replicate infinite life for a part on a vehicle. After all, these vehicles are expected to last for many years with minimal maintenance. By obtaining this figure, you can determine the part’s lifecycle management as part of ongoing maintenance.
With this expert design, manufacturing and testing, military vehicles are able to carry out work over a long time span, but they must also be able to cope with a multitude of scenarios, including deep water wading. To successfully navigate through water, the components underneath the vehicle must be completely sealed off to external elements to keep dust, grit and salt out of components, as well as keeping lubrication in.
This is where environmental testing comes in. For applications like this, this testing is arguably just as important as the physical testing. It is vital that every part of the steering system is able to handle water exposure, changes to temperature and humidity variance.
According to Emma Cygan, a Design and Development Engineer at Pailton Engineering, the environmental testing process for steering systems needs to be bespoke to the vehicle and end application. For example, a military vehicle used for semi-submersed and wading applications will have different requirements than an armored vehicle designed for warfare. Using adaptability in the testing process means testing is not confined or restricted to standard tests, but instead, bespoke tests should be arranged based on the end application.
Every test is designed to replicate the real conditions a vehicle will face, to ensure the vehicle is fit for purpose. While all military vehicles can differ, when it comes to the environmental testing of military-grade steering components, there are some crucial parameters to consider that form three core tests:
Salt Spray Test
Pailton Engineering uses a salt spray test rig, where up to six steering parts at a time can be tested dynamically against salt spray, at varying temperatures. This rig is a large container with a rotary arm that controls the movement of steering components, normally at a rate of one cycle every three seconds — one cycle representing one turn of the steering wheel.
The salt spray test does not necessarily represent dirt or grit but represents any debris that could be on the road. This debris could enter a part and cause it to corrode or fail, so the testing process is used to identify any potential problem areas. The test can use varying concentrations of salt, with higher concentrations being for the more extreme applications, as often seen in the military vehicle industry.
Salt spray tests can take on many forms, depending on the needs of the vehicle. For example, a test could check the effects of repetitive salt spray, with periodic temperatures of minus 40 degrees Celsius— this is the automotive industry standard for low temperature testing. The exposure to the requested temperature can vary depending on the customers’ requirements. This will ensure the steering system can withstand grit on the roads, even at low temperatures.
So, what can you expect to see from a test like this? The results will illustrate changes in torque, rate of corrosion, overall effect of grit on the vehicle and its steering system, as well as any potential for water ingression. Of course, the best possible result for this test to show is that these parts are capable of working in these conditions at low temperatures.
The same test rig can also be used to test the system against other factors, such as water. During rotary submersion, the parts are fully submerged in water. Ultimately, if a part can endure underwater movement at one cycle every three seconds, at varying temperatures, without corroding or failing, then the vehicle manufacture can be confident in putting those steering parts in a military vehicle for use in the end environment.
There are alternatives that can be explored to improve the performance of a part. From different finishes, to upgraded sealing and greases, there has been plenty of scope for development in recent years. With steering components taking on new and improved design features and passing rotary submersion testing, comes new opportunities for high performance extreme vehicles taking on deep water wading.
Low Temperature Evaluation
The importance of low temperature testing has been alluded to in both salt spray testing and rotary submersion, but it also forms a standalone test for military steering systems. Low temperatures can bring about issues with the viscosity of the fluid in lubricated components, which can have detrimental effects on steering torque, or even cause parts to lock up entirely.
Using a low temperature chamber, parts can be tested at temperatures as low as -40° Celsius, for a set period of time based on the data of the end application. This low temperature could be applied constantly, or periodically for more dynamic testing. The parameters measured in this test include assessing changes to the part caused by low temperatures, checking rotation, and measuring torque.
One customer's request led to the production of the next generation of bevel boxes at Pailton Engineering, needed for vehicles carrying out long journeys at low temperatures with high levels of moisture and grit. After improving on the design features of the previous bevel box, with a serration cover and alternative grease, the bevel box took on environmental testing. Operated at a rate of one rotary cycle every three seconds continuously and submitted to salt exposure and temperatures of -40° Celsius for four hours every week, the new design features were certainly validated.
External validation showed the generation three bevel box conformed with ingress protection codes IP66 and IP67, which is great news for the military industry.
Those who don't opt for environmentally tested steering systems may face problems with water ingression, stiff steering and general maintenance. There are plenty of variables to consider when developing a steering system for military applications and the testing process should always be unique.
This article was written by Eric Sonahee, Design and Development Engineer, Pailton Engineering (Coventry, UK). For more information, visit here .