Notifications
Clear all

Load Cell Selection

(@six_shooter)
Member

So I did a search on this forum to see if I could find the answer and still didn't come up with what I'm looking for.

I will be retrofitting a Land & Sea DYNOmite chassis dyno with YourDyno electronics and software. I will be ordering a load cell at the same time, especially after reading through a topic in my searching that suggests that L&S uses a strain gauge that is resistance based and therefore not directly compatible with the YourDyno electronics.

So using the calculator on this site, I'm not sure what to put in for the Brake RPM @ max power. Would this be based on the RPM of the shaft through the brake, so for an engine dyno this would be engine RPM, but on a chassis dyno this will be somewhere between 3 and 4 times slower than ENGINE RPM, based on final gear ratio... and then tire diameter and roller diameter as I'm thinking about this now... and would be RPM of the rollers? I think I have some measuring and calculating to do... lol In writing this I think I answered my own question, being that you're looking at RPM of the shaft through the brake, though I'm going to leave the question as is and ask anyway.

I would imagine that choosing a load cell would be best to where the range being used would allow to use a larger portion of the range to allow for better accuracy, right?

I.E. If I was going to be measuring 600 HP (at the tires), that I would be better to use a 1000 kg load cell over a 2000 kg load cell if the calculation suggested that the load would be in the 800 kg range, though I'm sure the accuracy would be close with either. 

Right now I am getting results of 3583-ish kg (based on 2000 HP total, which is what the dyno was originally claimed to be able to measure up to), if the RPM is around 2000 RPM and an arm length of 200mm, to as little as 1195-ish if the RPM was 6000. Most engines we will be measuring will be sub 1000 HP, but at least one will be close to 4 digits for the end goal, if not above. At 1000 HP I get 1792-ish and 597-ish, depending on RPM. I might see if I can fit a longer arm, but I'm not sure that will be feasible, due to where everything fits now.

I don't mind ordering a couple different load cells to have different ranges, but would like to not need to experiment too much and order a few times. (Shipping I'm sure will be enough shipping to me in Canada once, let alone a few times. lol )

Also from what I had read it seems that either the load cell needs to be installed differently between one dyno direction to the other or two different calibrations need to be used. Is that correct? Also any real issue with putting the load cell under tension? I would assume it would be more desirable to install it under compression, correct?

Also what happens if you exceed the ratings of the load cell?

Thank you

Quote
Topic starter Posted : 19/10/2021 5:26 am
(@thdyno)
Member

Hi.

Now im not an expert 

But i think the loadcell does not have to be able to measure more than what your brake is able to hold (nm)

Then you have to calculate how much that is with the lenght of arm you have from center of brake to center of load cell

ReplyQuote
Posted : 19/10/2021 3:00 pm
(@admin)
Member Admin

Hi there!

The easiest way to calculate the correct load cell size is use the max torque rating of the brake. You don't need a larger load cell, no matter what horsepower you will test, because that is max torque the brake can brake. Here is the formula:

Load cell size (kg) = Max brake torque (Nm) / 9.81 / load arm length (m)

Load arm length is the distance from the middle of the brake axle to the position of the load cell. 

Load cells work within the specs up to 100% of their rating. Up to 125% of their rating is no problem, but it may be slightly out of spec. Guaranteed survival is 150% of the rating, but you may need to recalibrate. More than 150% and you may damage the load cell.

It is best to choose a cell that is just about big enough. All accuracy specifications scale with the size of the load cell, because all specifications are given as a percentage of full scale. In practice there are much larger variations in the engine/drive train than in the load cell, so no need to be very concerned here. 

Yes, you cannot use the original Land & Sea torque cell, it is not compatible (at least the ones I have seen). One way is to cut the load arm in two and mount a bar type load cell where you cut it. Or you can mount a regular S-type load cell at the end of the arm. 

Load cells work in both directions. There is an option to reverse the calibration if the dyno turns in different directions for different cars/engines. Note that when you use this option, the load cell operates far away from where it is calibrated, so this is not ideal (puts higher demand on the linearity performance). If your dyno only turns one way, make sure you can calibrate the load cell the same way it is used; tension or compression (most common). 

 

ReplyQuote
Posted : 19/10/2021 9:11 pm
(@six_shooter)
Member

Thanks Admin...

So I can be pretty confident with the smaller load cell then, especially if they can read up to 150% of rated capacity without being (too) concerned.

I can see me ordering more load cells in the future to find best accuracy... and I'm the tinkering type. lol

I'm going to try and work on a lever that places the load cell under compression regardless of roller direction... that will probably be a later update though, since most of the cars we will be running will be RWD, and the FWD cars will be lower powered anyway, so a standard load install should be fine.

I would assume the roller weight (MOI) will affect how much force is applied to the load cell, since some of the torque will be used to accelerate the rollers, correct?

Just to clarify, your calculation above specifies meters (m), whereas the calculator on the instrumentation kit page specifies millimeters (mm), I assume the calculator takes this into account? I only ask because the result is vastly different if the numbers are different by a factor of 1000. lol

ReplyQuote
Topic starter Posted : 20/10/2021 4:24 pm
(@admin)
Member Admin

Correct, the MOI will affect the load that the load cell sees during a sweep. But your load cell should non-the-less tolerate the full torque of the brake. For example to measure the MOI you can do a ramp up then a ramp down, and during the ramp down the brake sees both the engine torque and the effect of slowing down the rollers. You may also want to quickly stop the rollers after a run, and the load cell will see that, even though we don't care about the reading.

Yes, sure the factor 1000 is applied when using mm 😉

ReplyQuote
Posted : 20/10/2021 6:18 pm
(@six_shooter)
Member

Made some progress. We spent a few hours measuring and laying out the dyno rollers location in the shop yesterday.

I've also decided to buy the trigger wheel and sensor after some discussion with Jostein, where he indicated that the L&S trigger is too low of a resolution to work with the Your Dyno hardware. I'm also going to grab just one load cell, a 1000 kg one, because the calculator indicates that at most maximums for the power we hope to measure, should be between 700 and 1300 kg, and the 1300 kg would be a pie in the sky measurement. Also with everything I'm ordering, I won't have enough credit on my card to order two load cells. LOL 

ReplyQuote
Topic starter Posted : 26/10/2021 2:46 am
(@six_shooter)
Member

It seems my replies aren't making it through...

How do you attach a picture on this forum? The "attach file" button doesn't seem to work for me, and I've made sure the images are below the 10mb maximum.

ReplyQuote
Topic starter Posted : 26/10/2021 2:47 am
(@six_shooter)
Member

So ignore my last reply, apparently it took some time for my original reply to show up on the forum... *shrug*

ReplyQuote
Topic starter Posted : 26/10/2021 3:15 am
(@six_shooter)
Member

Picture of the roller unit:

ReplyQuote
Topic starter Posted : 26/10/2021 3:17 am
(@six_shooter)
Member

How long are the cables on the load cell and RPM sensors? I ask, because if I need to extend them, I would like to order the connectors to be able to extend them when I order the other parts.

ReplyQuote
Topic starter Posted : 01/11/2021 2:29 am
(@six_shooter)
Member

I looked at the description again for the load cell and see that it says it has a 3.5M cable... not sure how I missed that one. *facepalm* lol

I assume the RPM sensor is similar?

Both look to be just wires at the end, I thought the load cell had a connector on the end of it, so if it's just wires, then I can extend those pretty easy. I don't think 3.5M is going to be quite long enough.

ReplyQuote
Topic starter Posted : 01/11/2021 3:36 pm
Share:
Select your currency
EUR Euro
USD United States (US) dollar