Preliminary design for alternative to bathroom scale lever.

[sporqster]

I'm actually working on this with a 1400lb max working weight (per wheel)- so you could use this to set up your pickup truck if you wanted to. The load cell is basically a surplus industrial single-acting hydraulic cylinder with a gauge stuck in the port. That is then inside a piece of tubing with just a slightly larger ID than the OD of the cylinder. The weight of the wheel functions like a scissor jack to compress the cylinder, resulting in a pressure reading.

 

Based on what I've gathered from talking to the guys in the lab where I work, I should be able to get around +- 5% readings on the weight, most of the error in the cheap pressure gauge I'm looking at.

 

I'm eyeballing the total cost of this to be about $50 each, $200 for the set of 4, in materials. Not uber cheap, but heckuvalot cheaper than a set of longacre's.

 

Has anyone seen or attempted a rig like this? feedback? experiences? any reason you can think of that this is a really dumb idea? I'm sure someone has thought of doing something like this before, and I tend to think there must be a reason nothing like this is commercially available (to my knowlege).

http://www.savannahoosiers.com/CHEAPSCL.JPG

http://www.savannahoosiers.com/EXPLOSION.JPG

 

[BobDrye]

I remember back in my sprint car days that we used a device that hooked to the lip of a front rim. I was based on a 1 square inch bore with a gauge that read the front weight directly. It was used for quick readings in the pit.

[Birkin42]

I have a few concerns with your design, if I am understanding it correctly.

My concerns are with the friction in the joints, stiction in the cylinder, and most importantly, the rising rate geometry of the scissors. As you compress the scissor, the tension force on the cylinder will increase at an every increasing rate. So for example, if the scissor is compressed to say 5" and you have 500 lbs on it, you may have a pressure reading of 1000 psi. If the scissor was compressed further (less oil in the cylinder) to say 4" and you have the same 500 lb load on it, the pressure may now read 1500 psi.

I think it will be hard to calculate the actual weight taking into account the force amplification/reduction in the scissor. Even if you only want to compare one corner to another and don't care about actual weight, you would need to verify that the scissor geometry is near identical at the corners you are comparing.

Unfortunately pivoting links that this issue. Some times it is a benefit, some times not.

 

Am I understanding your mechanism correctly?

[xflow7]

I remember back in my sprint car days that we used a device that hooked to the lip of a front rim. I was based on a 1 square inch bore with a gauge that read the front weight directly. It was used for quick readings in the pit.

 

I know exactly what you're referring to. Pegasus used to carry them (and for a pretty reasonable price, IIRC), but I can't find them on their site anymore.

 

Did you find they gave useful accuracy? I always figured they'd work okay on a softly sprung car but that on a stiffly sprung car, you would significantly alter the apparent corner weight by the time you lifted the wheel enough to unload the tire sidewall.

[sporqster]

Am I understanding your mechanism correctly?

 

Yes and no.

 

The green cylinder is full of hydraulic fluid, so assuming it's bled properly, it better not move at all. The assembly as shown applies has a TAN(41.09deg) multiplication factor on the cylinder (I didn't expect you could eyeball that angle ). I didn't totally pull that angle out of my butt. I picked this angle such that the the 7/8" Diameter cylinder with a 140MPA gauge would actually output a multiple of 10 of the weight on it - so when the gauge reads 35MPA, it is really displaying a 350 pound weight - get it?

 

The static friction and binding is a concern of mine as well - which is part of why I'm using 1/4" steel plate for the linkages - should prevent binding a little. But since the assembly doesn't move, as long as the forces are transmitted into the pins, they don't need to move (well, can't move really). I slotted the tube so that I can calibrate the assembly such that I can nail the 10X weight in pounds, shown on the MPA scale gauge. (this is done by slotting the holes on the upper 1" square tube, and connecting the top with rod ends LH and RH threaded... haven't modeled that part yet.

 

The cylinder is rated for 2250psi under operating conditions (7/8" Dia cyl, so 1350lbf) divide by that the TAN(41.09) and you've got 'round about 1500 lbs you can put on the scale without maxing out the cylinder. Of course the gauge is maxed out at 1400lbs. 1400lbs on one wheel is a dadgum heavy automobile.

 

 

[sporqster]

I remember back in my sprint car days that we used a device that hooked to the lip of a front rim. I was based on a 1 square inch bore with a gauge that read the front weight directly. It was used for quick readings in the pit.

Like this?

http://www.speedwaymotors.com/p/230,234_Hydraulic-Wheel-Load-Checker.html?itemNo=weight%20checker

 

[slngsht]

alright, here is a stupid question... why not just make a very short cylinder (maybe 3 or 4" tall), drop it in a small up/down ramp, and call it a day... no linkages needed.

 

It will have a direct ratio, so if you select the diameter of 1.78", 1000 lbs = 100 psi, etc...

 

It'd be much cheaper to build.

[jbanker]

I have the 0-600 lb Speedway Hydraulic-Wheel-Load-Checker and it works fantastic. Good investment for $160.

 

Jack

[xflow7]

Like this?

http://www.speedwaymotors.com/p/230,234_Hydraulic-Wheel-Load-Checker.html?itemNo=weight%20checker

 

That's the one I remember seeing.

[xromad]

I like it!

 

Wow! I thought I had seen every possible thing speedway had to sell.... And then you show me this.