G-Force limit on Trailer Mounted Diesel Generators?

[CobusBW]

Hi guys,

I've got a fleet of diesel generators mounted on trailers, these are rental units used for emergency or temporary power supply, often used in remote regions with harsh road conditions like gravel, mud, potholes and unpaved roads. In addition, we also manufacture trailer-mounted generators for clients looking for a mobile solution.

Sizes differ from 10kVA to 250kVA, all in soundproof enclosures, and combined trailer-laden weight is approximately 1500kg to 4000kg. All trailers have brake systems and blade springs for suspension, blade springs are rated according to the designed laden weight. Some use single axles and we have double axles on larger units, no shock absorbers are fitted on the trailers. The generators are mounted onto a sturdy base fuel tank via anti-vibration rubber mountings, the failsafe type, all rated for the specific generator weight. Soundproof enclosures are also bolted down onto the rigid base tank. The whole assembly then bolts onto the trailer directly.

Most equipment such as PLCs, switchgear, batteries etc could handle large G-Loads. The trailer however as well as the anti-vibration rubber mounts has its limitations. My suppliers cannot advise on the exact limits that could safely be exerted onto these items before failure could occur. We've had several instances of stress-induced fractures on the trailers as well as sheared anti-vibration mountings. Often one can see where the axles bottomed out during these events where there's clear evidence of axle centre bolt marks on the trailer frame.

I've tested G-Loads recently with a datalogger when towing a generator, hard acceleration, hard stops, a few potholes and speedbumps, but this was all in the city on tarred roads, readings differ from 0.2 up to 0.6Gs. Off-road conditions would differ, same with rural roads and uneven loads when one axle carries more load than the other if the trailer hitch on the vehicle is too high or too low.

As generators go out frequently and often we have very little time to inspect a machine before sending it out again G-Load Dataloggers would not be a practical option to indicate possible damage that might require further investigation. I'm considering adding Shockwatch Impact Indicator stickers to all our units as well as retail units as a visual indication of potential damage. Unfortunately, I'm not sure what to go for, 1G, 2Gs, 5Gs? I'm an Electronic and Electrical engineer and have no idea about the Mechanical Engineering aspects and what would be considered "Safe" limitations.

If anyone could provide some insight or advice on what limits to work with I would truly appreciate it!

Thanks in advance.

 

[GregLocock]

I suggest you figure out the actual specs of the anti vibration mounts, these seem to be a big clue.

You'll easily see 2 or 3g with that (hopeless) suspension setup.

 

[EdStainless]

We used to ship shockwatch sensors in our equipment boxes.
Usually a 2g and a 5g (though sometimes it was a 10g).
They would either come back with all of the sensors intact or they would all be broken to fragments.
Once trucks go off of regular roads all bets are off.
At the least you need bump stops, but shocks would be big benefit.

 

[GregLocock]

Even racing cars on circuits see 3g, production cars on durability roads see much more than that. And that's with proper suspensions.

 

[PersonalProfile]

The Load Restraint Guide for Australia states ~ 1 G:

https://www.nhvr.gov.au/files/202112-1285-load-restraint-guide-2018.pdf

 

[CobusBW]

Thank you for the replies!

Unfortunately, the trailers can't be equipped with shock absorbers, there's no provision from the axle suppliers or trailer manufacturers to add shock absorbers. Bump stops could be added (although not a manufacturer option), but ideally, the axle travel shouldn't travel that much. All the trailers are built to comply with local standards and I have yet to see one with shock absorbers or bump stops. Even on our larger trailers with Rubber Torsion Bar axles we often see similar damage. All these trailers are designed for slow-speed towing as the weight often exceeds the towing vehicle weight, these are similar to agricultural trailers but without rigid axles. To manufacture high-speed trailers with custom suspensions would not be economically viable.

Most of our clients comply with local laws regarding towing speeds and safe practices, but you get the other 5% who don't care. The latter is my biggest concern as these result in damage.

My suppliers are unable to advise on rating limitations on components, thus I have to determine this myself. From the comments, it seems likely that I should anticipate anything from 2G up to 5G. I guess the best way to find out would be to fit several Shockwatches and monitor them, at least this would give me a visual indication of when to anticipate damage.

 

[LittleInch]

My guess, for what its worth is that your weak point is the attachment of the rubber vibration mounts ( a picture of one would be nice).

As such the equipment / engine supplier is likely to rate these are a FoS of probably 3, so 3 times the weight of the machine / load on any one point. So a 2G sensor is the lower end and 5G definely the higher end.

That sort of load though will also bend the wheel or the axle.