Weighing some large vehicles.

load cells?

They would probably need to be installed in the supporting structure before the vehicles are installed. Is this a new facility? Are the vehicles new to the facility?

Is this a one time weighing of the vehicles? or are they weighed once a day, a week, or ?

My experiences with load cells was about 20 years ago. We were weighing large hoppers that were filled with coal. We also weighed material moving over conveyor belts.

For such weights the solution is to lift them on hydralic jacks, equiped with load cells if you need a higher degree of accuracy.

I designed build and tested systems for weights up to 4000 T (off shore modules) the uncertainty was less 1/1000 of measured value even at loads around 1/5 of the nominal.

If the uncertainty requirement is low there are less complex and less expensive methods.

It is necessary to know more about the vehicles and the accuracy requirements to give you a correct answer.

Any way what you intend by lifting one side at a time leads to quite important errors so that it can be used ONLY if the uncertainty requirements are low.

It will be a pleasure to give you the hints you need.

First time I have used this service. It would be easier if hitting "respond" did not take away all earlier comments. Having them for immediate reference would be beneficial.

So, lets try this and test my memory. The vehicles involved are not new. some in fact are over sixty years old. A count would say there are effectively seven of them. Of those, two are single assembly and therefore might be counted as one, if that mattered. It does matter in that it the weightof the smaller one (which rides on the bridge) makes achieving appropriate levels of uncertainty a different challenge. I will work on establishing what is an apprpriate level of uncertainty, but off hand I suspect that one part in a thousand is sufficient.

As noted previously, these devices each sit straddle a body of water of a width between seven and a hundred meters. It was becasue of that separation that I start with preseumption of weighing either side separately, but I see that I have not looked at how splitting measures will impact uncertainty. (We are a research facility. Uncertainty will be an issue of some importance.) Any idea how much impact it would have? (Double?)

The vehicles are open steel frame structures of various designs. They are are structurally quite stiff and enduring.

We are the David Taylor Model Basin, at Carderock, MD. These facilities include the tow tanks. (The link to the shallow water basin provides the best view of the carriage.) They also include the Maneuvering and Sea Keeping facility (a large wave faciilty. in the picture see the bridge and the little carriage under the bridge) and the Rotating Arm. Each has links to technical desccriptions and basic schematics. This should give a better understanding.

There is no record that they have been weighed in the past. The desire is to weigh them and be able to repeat it whenever significant changes occur.

What else can I tell you?

Based on your explanations I can give some input since i know now the structures: i developed for a research enter in Europe transducers for similar research facilities.

For such structures (≈100 m) the influence of the lifting on one side is too small to be considered. In fact the errors are more due to the shift of the COG projection and is more important in relatively high structures. Which is the ratio distance between wheels/distance between rails? if is is small then you can lift on one side but all wheels at same time. i.e. lift at the 2 corners over a rail. If there are more wheels the problem is more complex because several cylinders and associated load cells have to be used. Since load is not uniform teh cylinders have to be supplied each one with a separate flow in order to avoid a higher lifting where load is less important. The height of lifting should be as small as possible and only allow the load transfer on the load cells you should put on the rams. In case of sevetal wheels every one has to be checked for contact. The better is to use high pressure rams (700 bar for instance) and short stroke with low flow pumps. The load cells must be equipped with tilting saddles in order to avoid force eccentricity. The rams capacity should be at least 2x expected load. The same for the load cells.

The suggestion with strain gages on the rail is good but it has to be calibrated, which means you have to put a carriage with a known weight (measured with very low uncertainty) and correlate the signal of the strain gages (you MUST build up a full bridge) with the position of the wheel with respect to the section where the strain gage is bonded. The rail is an undetermined structure and the own stiffness has a strong influence. Since your structures are in an humid environment the strain gages have to be very well protected if you want to use the system for a long time). For an uncertainty reduction you should record the ambient temperature at each measurement since the Young modulus is modified by temperature (for steel ≈ 350 ppm/°K). At least for the 1st measurement it would be better to use calibrated load cells with a temperature compensation. It has to be taken into consideration that the best strain gage bridge can modify in time so that from time to time a check with known weights has to be done (periodical calibration).

Strain gages can be found by Vishay or a distributor of Vishay I am sure some of your coworkers in the lab use them for special equipments.

If you need more detailed informations I should have a possibility to directly give it to you since i doubt that can be of interest for all participants.

The people that I would strongly recommend you to get in touch with is Weighwell Engineering Ltd in the UK www.weighwell.co.uk who I have worked with in the past and also operate worldwide.

BEEJ50, Thanks for the suggestion. I have sent email to them. It is certainly promissing, although I am sure our installation will be unlike any they have run into in the past, it does have some similarity to rail0mounted cranes.

I remain open to other ideas, suggestions and discussion. Anyone know of other sources of this type of service?

Best Regards,

Sam Hicks.My intention is only to help.Are the veicules on rails, aren`t they? Is it possible to move them forth and back?If it is possible, my idea is to place strain gauges over the rails (special ones, with a rugged structure),move the veicules over them and make the readings on the display connected to the strain gauges through electrical wiresThis is like a brain storm, isn`t.

Yes, it is very much like a brain storm. I appreciate your input. It seems promissing.

I am familiar with use of strain gages on shafts to measure torque as reflected in the rotation of one part of the shaft relative to another. Also as used on a loaded member to measue elongation or compression with longitudinal loading. This would be a special sort of strain gage, to measure the deformation of the rail in response to the wheel passing over. (Proportional to the square of the modulus of steel - 30 million squared. Bunch of force.) Any idea where such strain gages could be found? Who makes them?

Seems to me if done this way, all wheels could be done at once and thereby derive total vehicle weight from the measurements.

Leaves a bunch of questions.

Thanks, again.

Thanks to you all for being interested in my situation enough to help. It is a bit like having an internation conference on the subject. I am more than impressed with it.

I want you to know that I am acting on the information and considerations you have pointed out. To that end I identified over 300 potential sources of support, either with products, service or both. On review (very quick) that was narrowed to less than 25, and of those I have sent email request for assist/information to four. I hope to have responsees tomorrow.

I expect to find aspects of these situations beyond my knolwledge and experience and hope it is acceptable to you for me to email you through CR4 email service.

Best Regards (for now)

I can vouch for instrumented rails or instrumented wheels. In the rail industry we use this method quite a lot, both for weighing vehciles and for detecting wheel defects in trains at full line speed. Our test centre has set up systems for this is sweden, we completed nother one this year for measuring iron ore wagons with an axle load of 30T, the joy of strain guages is so long as the rails you are on are still behaving elastically at higher loads, then you will still be measuring fine... if they are not behaving elastically, then your equipment will end up in the pool :) Another method is using Intrumented wheelsets, we are one of the few companies in the world that can do this, we instrument railway vehicles wheelsets to measure vertical and lateral forces, you could take the same approach with your machines. One advantage is that the wheels are calibrated here in our lab, so you skip the challenges of instrumeting the track, another thought is that you can use them when your equipment is rolling, which can tell you the dynamics of your machine, and also help you monitor safety, and keep ahead of rail/vehicle damage. If your interested gregory.riggall@interfleet.se