Leaky Wheels

[ozzie tractor]

sorry to say it but i have seen a few large kubota front rims crack at the lug for the spoke, (no not mine) it usually occurs on machines which are getting a lot of heavy FEL use. an M90 was the last one i repaired. i welded up the cracks, plus the oness which where forming around the wheel stud holes in the centre of the wheel. welding seems to fix the problem as long as the guy doing the job can weld properly and they make it a reasonably solid sort of a weld. yes they are tubless. no 25 psi will not hurt. i have personally run 30psi a few times when lifting seriously heavy things over the years, its the only way to keep the rim of the ground.

 

[have_blue]

sorry to say it but i have seen a few large kubota front rims crack at the lug for the spoke, (no not mine) it usually occurs on machines which are getting a lot of heavy FEL use. an M90 was the last one i repaired. i welded up the cracks, plus the oness which where forming around the wheel stud holes in the centre of the wheel. welding seems to fix the problem as long as the guy doing the job can weld properly and they make it a reasonably solid sort of a weld. yes they are tubless. no 25 psi will not hurt. i have personally run 30psi a few times when lifting seriously heavy things over the years, its the only way to keep the rim of the ground.

Ozzie,

Yours is exactly the type of info what I was hoping to learn. Thank you!

We do a lot of heavy FEL work, and keep the tires overinflated. Sidewall max calls for 18psi, and we use 25psi because it helps firm up the squishy ag tires. The tractor has never been ran over stumps, rocks, curbs, or potholes, and very seldom roaded.

FYI, Kubota is replacing the 2 front rims under warranty.

Haven't looked inside the wheels, but judging from the outside, they are a leak waiting to happen. The spokes appear to be riveted to the rims, which (it seems to me) are bound to loosen or cause cracking. Perhaps the rivets are welded inside the rims, which would be an improvement, but still a source for cracks to form.

Thanks again Ozzie!

 

[Soundguy]

I'm not talking about a static load.. I'm talking about a dynamic shock load. I also figure there is a max expansion point for that 'squishy' tire, where it will no longer 'expand to maintain exact internal volume.. once that is reached.. pressure goes up.

Take a hyd ram ( on a tractor ) happilly supporting it's max rated weight.. not have that tractor drive over real bumpy ground.. have a wheel fall in a deep hole, and get that max static load moving, then stop.. I've seen hoses pop from spikes.. and I've seen packing blow out, and cyl walls rupture.

Take a basketball and run over it with a tractor... I can tell you for sure that the outside surface contact area goes way up.. internal volume goes way down.. then you here a poof... I um.. have expiremented on this one...

I still say that air pressure variations from within a sealed container can spike when you abruptly deform the container so as to reduce the internal volume. Keep inmind you can deform the container faster than the container can plastically deform/expand to allow for more internal volume.

Another good example.. a well pump with a pressure tank that uses an air bladder. Water is pumped into the tank.. bladder has a set internal air volume, and static air pressure in it.. you pump in water and compress the bladder.. making the tank build water pressure.

I guarantee you it is not the water that is compressing....

Soundguy

ok few things hear...

a tire is a gass filled rubber container able to change its shape but not its volume (with in reason). a hard rubber ball is a solid. meaning you can sweeze its shape and affect internal pressure. your correct that its easy to change the volume of a squishy shape, but you have to apply that pressure evenly on all sides for that to work, else your squishy shape is going to buldge out on sides that dont have that pressure applied.

a heavly loaded tire in contact with a hard surfce changes shape because thats the only way it can continue to support the load on the wheel. its Pounds per sq inch. so if i reduce the area of contact (with an edge of curb) the only way to maintian the load is to spred it out more so the tire changes shape to create more surface to bear on ..

an example is good

say normal resting weight a) on tire pressure b) on a 2x3" sq patch of rubber.

roll up to a curb with edge contact of 1" then the tire has to get wider to support the load... it has to "squish" to 6" wide (the example is a bit over simplified because as you squish the tire it makes more contact with the curb so the "contact" area of what was org 1" grows as well as the width)

another way to look at it, 2x3" patch of rubber supports a given load at a given pressure. you load the FEL up and now look at the tires. they squat more. why? well in order to support the larger load they had to create more surface area. load(lbs) = area(sq inchs) * pressure. (lbs/sqinch)

still dont belive me, at home tonight, measure your pressure to the nearest pound. (needs to be accurate) then load up the FEL and measure it again.
you wont see an increase in the pressure.

to get all mathy on ya check out the law that is the reason behind the above examples. its called Boyles law Boyle's law - Wikipedia, the free encyclopedia

Boyle's Law states that the product of the volume and pressure of a fixed quantity of an ideal gas is constant, given constant temperature. Expressed mathematically, the formula for Boyle's law is: p V = k

NOTE: (while air isnt an ideal gas you can consider it one for our every day lives. also note that should you heat or cool your tires the pressure WILL change inside)

 

[tydp]

FYI, Kubota is replacing the 2 front rims under warranty.

Haven't looked inside the wheels, but judging from the outside, they are a leak waiting to happen. The spokes appear to be riveted to the rims, which (it seems to me) are bound to loosen or cause cracking. Perhaps the rivets are welded inside the rims, which would be an improvement, but still a source for cracks to form.

I beleive they are spot welded.

 

[schmism]

To prove the point I even went out back and checked the tire preasure on a loaded 18 wheeler I have on my lot. With out a trailer the rear tire preasure was 121.2 PSI. With a loaded 53 foot trailer (close to 40,000 pounds as it set's) on the 5th wheel the preasure went up to 122.4 PSI. I used my Snap On Vantage graphing meter with a digital preasure gauge that is accurate to 1/100 of 1 PSI to take the preasure measurement.

yes your correct, a tire doesnt have an infinit ablity to expand to distrubute the load.
but an increase of (1.2psi) .980% increase air pressure is hardly a "spike"

Another good example.. a well pump with a pressure tank that uses an air bladder. Water is pumped into the tank.. bladder has a set internal air volume, and static air pressure in it.. you pump in water and compress the bladder.. making the tank build water pressure.

I guarantee you it is not the water that is compressing....

the point your missing in this example is that the volume of the internal air is not constant. you are applying the pressure to the containter equally on all sides thus decreasing its volume, thus increasing the pressure in the container (THATS WHY IT WORKS!) (the steel skin of the tank applies equal pressure to the "non-water" side of the air blatter)

look at it this way... blow a ballon up half way. now useing 1 hand and a surface sqeeze, compress, bend, controt, mash the ballon all you want. The ballon will ALWAYS conform to a diffrent shape that maintains the same internal pressure and therefore volume. (within reason) the same is true for a tire.

anyway good to hear that Kubota is replaceing the wheels.

 

[Soundguy]

The example of the semie tire.. was IMHO.. not given as an example of a 'spike'.. but rather a static load, showing that there are material deformity limits.. and that when the material can no longer deform, IF, it can maintain structural integrity, then it may start to have a decrease in internal volume. Same with your balloon example. You may find that at a point before the balloon pops, there is an increase in internal pressure.. especially in a 'spike' or dynamic load situation.

For sure you are familiar with the concept of localized pressure differences. A concussive blast wave ( from an explosive device, for example ) is an example of an unrestrained localized high pressure spike. There is no container holding the pressure at a set point.. yet, mere feet away from the headwall of the wave, the pressure may be normal.

Look at pump and diesel cavitation.. localized pressure differences caused by vibration, or vacume and vapor pressure.

By the way, just curious....What kind of an engineer did you say you were?

Soundguy

yes your correct, a tire doesnt have an infinit ablity to expand to distrubute the load.
but an increase of (1.2psi) .980% increase air pressure is hardly a "spike"



the point your missing in this example is that the volume of the internal air is not constant. you are applying the pressure to the containter equally on all sides thus decreasing its volume, thus increasing the pressure in the container (THATS WHY IT WORKS!) (the steel skin of the tank applies equal pressure to the "non-water" side of the air blatter)

look at it this way... blow a ballon up half way. now useing 1 hand and a surface sqeeze, compress, bend, controt, mash the ballon all you want. The ballon will ALWAYS conform to a diffrent shape that maintains the same internal pressure and therefore volume. (within reason) the same is true for a tire.

anyway good to hear that Kubota is replaceing the wheels.

 

[schmism]

Look at pump and diesel cavitation.. localized pressure differences caused by vibration, or vacume and vapor pressure.

By the way, just curious....What kind of an engineer did you say you were?

Soundguy

pressure wave fronts due to concusuive explosions and "air in a tire" are kind ain a diffrent relm of physics

i have a BSME with a minor in physics but work in the civil (construction) world

 

[Soundguy]

Yep.. but it's all realitive. once you get a container that can no longer defore.. but hasn't reaced it's failure point.. you can get decreased interior volume, and thus increased air pressure. Same if you have a sudden decrease in tire volume befor ethe tire can deform.. I.E. instantanious shock loads.

ever seen a loader hose blow when the loder operatr drops the bucket and then stops it at the last second... there was a pressure spike.. just like happens in a tire when it suddenly finds itself sufficiently smaller and either can't deform fast enough, or can no longer plastically deform..

I think this will be my last post on this subject.. it's getting kind of stale debating something that you can prove satisfactorilly with a mounted and inflated automobile tire, 25$ tire gauge with a peak reading holding feature and a sledge hammer.

Soundguy

 

[DieselPower]

 

[Tom_Veatch]

OK few things hear...

a tire is a gass filled rubber container able to change its shape but not its volume (with in reason).
...

Assuming external force applications, that's a big negative!

Assume the inner surface of the cross section of a tire is a circle. It's not really, but close enough. Of all closed curves with a fixed perimeter, a circle encloses the maximum area. That's why things under pressure bulge and try to assume a circular or spherical shape.

So, if you squash a circle, it begins to look more like an oval or an ellipse. Since it no longer approximates a circle but has the same perimeter as before, the enclosed area must decrease. Take it to the limit and completely collapse the figure so that the bottom touches the top and the sides expand out as far as they will go. The area within the figure just went to zero.

Applying that to a torus, the approximate shape of the inside of a tire, when you squash it without changing the perimeter of the cross section, you reduce the local volume within the squashed part. Some other part of the torus may bulge, stretch it's perimeter, and enlarge it's local volume. That's what happens to the balloon, and to a much lesser extent, to a tire.

Tires have all kinds of radial, bias, and/or circumferential reinforcement which strongly resists that stretching. I am reminded that tires don't stretch very easily everytime I have to dismount or mount a tire on the rim.

Same amount of air, less volume, more pressure.