About loads for testing a new vehicle

[pietro82]

Hi all,

It's well know loads on a vehicle are strongly dependent by vehicle parameters itself and they can be different markedly from a vehicle and a similar one (as example from the old mercedes c-class and the new one). It's also well known that measuring the loads on a prototype is time consuming. What is it commonly carried out from vehicle companies? Are testing loads measured anytime a new prototype is built in some conditions?, Are they computed from multibody simulations? Are they measured only once every a couple of new vehicle and correct somehow as example using the GCW?

thanks

 

[tbuelna]

pietro82-

Think of the situation in these terms- If you have say 36 months to design a new transmission, test it, and get it into production, do you think it would be possible to complete a 2500 hour lifecycle test on a dozen or so production conforming transmission test articles in that timeframe? Probably not.

Fatigue testing of transmissions usually involves testing at many different conditions, including accelerated testing at elevated load/speed. The risk of getting the design of a production transmission wrong can mean millions of dollars in warranty costs for the OEM. A Weibull analysis based on just 3 data points would not be satisfactory for an automotive OEM.

 

[pietro82]

Thanks guys.

so dozen is impracticable and 3 is not that much. So I believe the number of samples is dependent but the possible number of the different failure modes, that each one should be tested; in case of a transmission there are at least 4 failure modes (i.e. gear tooth bending, gear tooth contact, roller bearing, ball bearing) so I wuould believe 8 transmission samples is the min number, is it right?

 

[tbuelna]

pietro82-

The type of test you are asking about is called a "qualification test". The purpose of a qualification test is to validate the design and analysis work behind the device being tested. Things like auto transmissions are certified based primarily on critical reviews of analysis reports for things like stress, fatigue, dynamics, reliability, failure modes/effects, performance, etc. Due to the large number of potential failure modes a complex system might have, and/or the huge number of load cycles required to test an individual component to its predicted fatigue life limit, these variables are often only validated by analysis work.

If you're going to conduct a bonafide qualification test on a transmission, the test article should ideally conform to production standards. And this would require using transmission components that were produced using production materials and processes. If these conforming components are not yet available for the qual test article, then the only alternative is to use components that were designed, analyzed and manufactured using existing processes that were validated by previous qual tests.

The analysis and testing of something like a new auto transmission design is no small matter. Due to the slim profit margins the OEM makes on production auto transmissions, if their calculated reliability of the transmission is off by a small percentage, the warranty replacement costs can result in huge financial losses for the company.