The followings are questions collected from our customers, vendors, manufacturers and other interested or seriously involved parties, and answered by our team. Real people, real questions, real answers. Upon request, with the questioning party’s consent, the correspondence or transcript may be forwarded or copied to people with genuine interest in Air Suspension Wheels (ASWs).
Q1: How is the maintenance on the air cylinders is done? Is it expensive?
A1: Our air cylinders are hydraulic cylinders filled with nitrogen gas, experiencing much lower pressure then hydraulics do.
The compression-sealed Viton fill-valves are warranted for 10 years for zero leakage.
The piston and sleeve seals are similar. The aging, if any, shows up on the viper seal. Easy to inspect.
The swivel hinges are internally greased under spring load with 10 years grease supply.
The air cylinders are recommended to be checked any time when the hydraulics are checked on the vehicle during scheduled maintenance. Once a year approx.
The maintenance need is minor, so we assumed 4% sales price for that, used up over 10 years.
Q2: What about the traction? What kind of tread patterns are available to operate on ice, snow, and muddy roads?
A2: Several options are shown on this website. We design treads to the need.
Easier then rubber tire makers ever could. You cannot make hard rubber treads on inflated tires.
Soft or hard rubber bonded to steel inserts are bolted down individually.
Replaceable treads may be integrate but separate business from the steel wheel structure. Most treads can be made of steel or cast iron, of course. Diamond plates and spikes are no problem for us.
Q3: What about the lateral load and flexibility? The bigger the truck, the more dynamics enter the picture. On the oil sands for example, with the soft underfoot conditions, the lateral loading on the trucks and chassis is extreme. This concept to me makes me envision a bias tire of the same size—be pretty tough on the truck, I would suspect. I guess, I can see frame related problems with this concept. The trucks have enough on their own, don’t they?
A3: The steel air suspension wheel (ASW) have the same size, weight and deflection under the same load as the rubber tire have.
The ASW’s lateral stiffness is much higher when we use the air cylinder hinges without swivel eyes in conjunction with heavily stiffened side walls, the way that is shown on this website.
We can tailor however the lateral wheel stiffness to be the same or even softer than that of the rubber tire.
The rubber tire is vulnerable to collapse under excessive lateral load.
Our wheels cannot collapse, even if we configure it sideways softer than a rubber tire.
The damping of a rubber tire is negligible, which affects vehicle dynamics negatively.
The air cylinders are excellent nonlinear air springs and dampers . That reduces stresses and fatigue on the vehicle frame.
You want it laterally stiffer to help in cornering and stabilize driving or allow for higher speed, we make it that way.
You want it softer instead, to isolate lateral loads transferred from ground to frame, we make it that way.
You want balance, you’ve got it. It often involves only modifying the charge pressure in the air cylinders.
Q4: Now, for a loader or a grader or anything else that might be unsprang (!) might be a different story! No suspension required by this type of unit and the concept just would fit. Right?
A4: Our wheel, the ASW, can be engineered to be barely softer than a solid rubber tire or softer than a softly inflated (say deflated for sand crossing) tire. We can separate the lateral, vertical, forward, reverse and circular ( torsional) stiffness by design. All these are strongly coupled properties of the inflated tire and thus are hardly separable. We tailor the ASW to the application and need.
Replacing Pneumatic Tire Wheel (PTW) with Air Suspension Wheel (ASW) is illustrated with the diagram below, where W is the axle load as weight (gravity load), m1 is the weight of the rim assembly, m2 is the weight of the tire (drum assembly in case of the ASW), k is the spring constant of the wheel (equal wheel load divided by wheel the deflection under full load), a is the ground acceleration (for instance a 2g bump load) and m is the wheel weight (m=m1+m2).
For mining truck and wheel loaders, m/m1 is 15-45% (30% on average) and for PTW, it is 25-35% (avg. 30%) for ASW. The tire weight is transferred through the bead to the rim, thus m is concentrated at the hub center. The tire complies (deflects under) the road, thus k is between m and the ground (infinite mass). The drum is rigid (the road complies) and thus m2 is coupled to m1 through k. For the vehicle frame or chassis suspension (if it has one) and for the powertrain and brake assembly, the unsprung mass of the PTW is m (100% wheel weight) and m1 of the ASW (30% wheel weight). The drum of the ASW is directly supported by the ground. Thus m2 is decupled from the powertrain at vehicle acceleration and deceleration. Turning of the wheel is delayed via the elastic coupling of k.
The importance and the definition of sprung and unsprung weight of a vehicle is often misunderstood, even by engineers. In general, if the vehicle has rigid body and pneumatic tire wheels, the wheel mass and any and all masses under the chassis suspension is considered unsprung, for being under the spring of the suspension. Any reduction of such unsprung mass (components weight) improves drivability and reduces chassis vibration. In particular, the vehicles with in-wheel suspension, has a wheel mass fraction (m2) which is unsprung from the two-degree-of-freedom suspension system (wheel suspension plus chassis suspension), while m1 is unsprung from the chassis suspension. That means that an ASW replacing a PTW can be 3x heavier, without overloading the drivetrain or the chassis or the vehicle frame. That advantage seldom need to be taken advantage of, since replacement ASW matched the PTW size, deflection, softness (k) and weight. That alleviate vehicle warranty issues related to tire, rim or wheel selection.
If, as usual, under full load, a mining truck tire deflection is 7% of tire diameter D, so k=14W/D, then since the PTW is 19-21% of the unloaded truck weight and 8-12% of the loaded truck, and thus the mass participating in vehicle chassis or frame vibration is 7% less for unladen and 14% less for laden truck (10% avg.), the oscillation frequency of the vehicle is about 8% less for fully loaded and 4% less for unladen (empty) or about 6% less on average, which translates to 1.06^2-1=12% more comfortable ride with ASW. Driver’s fatigue is a safety issue, so this may translate to proportionately higher safety.
The diagram above shows a translational spring of spring constant k to illustrate wheel dynamics aligned with gravity. Illustrating rotational wheel dynamics is omitted. To calculate the rotational inertia of the compared wheels, the mass distribution for both the PTW and the ASW is about the same as shown above for the ASW, though, depending on configuration, the ASW may have up to 17% more rotational inertia. The main difference however is that the torsional spring constant of the ASW is 10-12x less than that of the PTW. Consequently, the ASW is more sluggish to brake or accelerate and better keeps its momentum, which is the vehicle speed multiplied by its weight. This property contributes to in-wheel energy accumulation, storage and return, which can result in fuel savings up to 16% and 10-12x less chance of vehicle skidding on ice, snow, mud and sand covered pavement or well-maintained dirt road.
Q5: You quoted 8 common threats to haul truck tires under the Problems Solved tab. Can you explain how you solve these one-by-one?
A5: 1) Sidewall cut: steel cannot be cut with knife (sabotage) or sharp rocks. Only the paint can be scratched that way.
2) Running in ruts: The ASW section is hermetically closed and watertight. The treads ensure uninterrupted mud flow across.
3) Underinflation: The air cylinder pressure is set for life. The cylinders are leak-proof for life (10 yrs. min.) No need for monitoring and altering cylinder pressure.
4) Excessive speed on rough haul roads: The air cylinders are designed for 2x full load (the dynamic load factor is 2). To develop that high load, the bump must be section height high at full speed. Air suspension wheels has no bead separation issues. The exponentially hardening elasticity affords extremely high impacts within short piston stroke, shorter than the deflection of the inflated tire under the same dynamic load.
5) Spilled material on the road: Same as 4.
6) Bumper blocks: Same as 4.
7) Windrows: Same as 4.
8) Dry steering: The stiffened section walls are bolted to the chord (drum) forming a rigid section to tolerate much higher steering forces than any inflated rubber tire can.