Dozers towing Haul Trucks

[mabn]

Hello fellow earthmovers,

I'm hoping to glean off some expertise if anyone has experience with the following:

We have a problem with haul trucks getting stuck in soft material.... or their tires spinning out as the truck starts up in the soft material at the loading area. We have limited the payload on the truck to help reduce this; at the limited payload, we havent had too much trouble.

We would like to put a dozer in front of the haul truck to help pull it out, or increase the payload limit that we can pull out from the loading area.

We are having some difficulty relating the drawbar pull of the tractor to the rimpull of the haul truck. How they extra pull power of the tractor relates to added payload on the truck is difficult to conceptualize.

As I see it, the factors involved are:
TR% - Total resistance of the ground (grade + rolling)
Ct- Coefficient of traction
Max. rimpull of the haul truck
Max. drawbar of the tractor
Payload
Empty vehicle weight of the haul truck
Tractor vehicle weight

I may be missing out on something?

The only variable that would change in the two scenarios (1. without the tractor, 2. with the tractor supporting) should be payload; and the increase is what I'd like to solve for.

The other variables should be constants. The TR% and Ct would be unknowns (but could be assumed if they cannot be solved for), the rest of the variables are knowns.

My approach would be to solve for TR% from the equation:
Max. truck rimpull = Gross Vehicle Weight with limited payload * TR%

Now I'd add in the tractor:
Increase the rimpull by directly adding the tractor drawbar.
Making the GVW an unknown (known vehicles weights plus unknown new payload).
Using the solved TR%.
Solving for payload, again using the formula above.

With this process I haven't used the Ct at all, and I'm really unsure whether I can directly add the drawbar to the rimpull??

If anyone has any input on this process or any ideas, it would be greatly appreciated. I want to take a textbook engineered approach to this before we test it in the field.

 

[digger242j]

I have a couple of thoughts, but first a disclaimer--I'm not an engineer, but as an excavating contractor I've spent a lot of time in the past 25 years playing in the mud. Also, I've not had any experience with *really* big equipment. (You list your discipline as "mining" so I'm assuming you're dealing with some large pieces.) I think it's safe to assume that the principles involved will be very much the same, regardless of scale.

"We have a problem with haul trucks getting stuck in soft material.... or their tires spinning out as the truck starts up in the soft material at the loading area. "

My interpretation of this is that you have two different problems, which manifest themselves in similar fashions.
Getting stuck is a result of rimpull insufficient to overcome your total resistance. Tire spinning is a result of traction insufficient to transfer all of your rimpull to the ground. Using a dozer to assist might solve the problem in either case, bit in different fashions.

"With this process I haven't used the Ct at all, and I'm really unsure whether I can directly add the drawbar to the rimpull?? "

A dozer will transfer "horsepower to the ground" over a range of tractive conditions, in a much different fashion than a wheeled vehicle. In my limited knowledge of the subject I believe that that would be expressed mathematically as a different coefficient of traction. I would venture to guess that close to 100% of the horsepower is available at ground level, so I'd further venture to guess that you'd be safe to add drawbar pull directly to rimpull.

In the case of "getting stuck" the dozer will add the horsepower needed to overcome your total resistance. In the case of "tires spinning" the dozer with it's greater coefficient of traction will compensate for the inadequate traction inherent in a wheeled vehicle.

I'm not sure whether those thoughts will be helpful, or muddy the waters further...

As an aside, I'd be interested to know from a logistical standpoint how the added effort of the dozer hooking up, pulling, stopping and unhooking, and repeating that cycle for every load balances out in terms of increased productivity. I imagine there must either be a reasonably long haul time or a fairly large increase in potential payload involved.

 

[treg]

Why not push rather than pull?

 

[DRC1]

The drive wheels of the truck develop a certian torque. If the ground is slipery but firm, such as snow on a road, the coefficent of friction between the tire and the snowlowers to a point where it can not produce sufficent tractive force. This is overcome by increasing the cofficent of friction, usually by using sand.
In the other case which I think is your case, the road material is sufficently soft that the torque from the drive wheel is sufficent to shear the soil, which is why the truck will sink as you increase throtle.
Pushing trucks can be done and most single body dumps are equiped to be pulled. The dozer wil transfer a line pull to the drawbar on the truck frame. That line pull is resisted by the resistance of the wheels in the mud. Because the truck wheels are sunk slightly in the the mud it must climb out, Thus determining analyticaly the required pull is rather difficult. I would check with who ever owns the trucks to be sure the frame is sufficent for towing, get a D-8 and try it. Two things to remember: Leave a long cable so the dozer is pulling from good ground, or you can quickly have two stuck vehicles. Make sure the towing cable is properly rated and in servicable condition. Make sure no one is standing near the cable during the pull. If the cable should part it could get ugly. Two reasons you don't want to push: 1. the dozer must be in the same soft ground as the truck which if the dozer does not get stuck will leave a royal mess for the next truck 2. Many truck do not have frames designed to be pushed and could be damaged by pushing.
As digger242j pointed out this approah will costly and time consuming. A more cost effective method may be to put down geotextile fabric and 2 feet of gravel. This can give you a reliable haul road on very pour soils.
One last note I would not plan on towing trailer dumps.

 

[mabn]

Thank you for all the input so far, it's helping me to clarify the problem.

Like DRC1 says, we can't really push the trucks because the dozer would likely damage the tires or the frame of the machine... sort of out of design specs. We haven't examined the geo-textile fabric with gravel, but I will definately look into it. The only issue I can see with that is the lack of portability. The digging face in a minesight changes locations quite rapidly.

Getting back to the required pull to move the truck. Would it not be the same regardless of whether the truck was pulled by a dozer or not? Hmmmm, unless you mean that the resistance would increase when payload increases, becasue the truck will 'sink' a little more?

As for digger242's question, we are looking at this scenario because our hopes are to significantly increase the payload of the haul truck.

Please, keep the thoughts/comments coming. Thanks

 

[Grouser]

How about preparing a centralized loading area (roadbed) for the trucks where you can control the "soil" conditions and get the material to be hauled from the face to the trucks with a short mobile conveyor that can easily follow the face excavation. You can then construct the haulroad bed so that sinking won't be a problem, it could be a separate operation that follows the excavation for the next pass at the face. Two conveyors would allow them to leap-frog so there would not be much down time as the excavation travels along the face. Gangs of conveyors would fill the trucks faster.

Good luck.

 

[Heckler]

Here is a list of variables that I see need to be addressed.

Grade Resistance
Rolling Resistance
Gradeability
Tractive effort
Rolling Radius of Loaded Driving Tires.

It also might help to know the CG of the haul truck both loaded and unloaded.

Best of luck

 

[mabn]

Grouser,

Good idea... a staging area would make a decent plan, except we are looking at 5000-8000 tons of material coming off of a face every hour. There are massive bucket wheel type excavators available that would accomplish just the thing you are talking about, at the production rates we need, except they haven't proven themselves very cost-effective from a maintenance standpoint. Also, once you've conveyed the material a certain distance, you might as well try to convey it all the way to the plant.

With smaller volumes though, a bucket-wheel/conveyor system would probably be the most cost-effective option. Get rid of the trucks and shovels completely.

One idea that a collegue mentioned was to use a slingshot type of setup, similar to that used on an aircraft carrier. The truck backs over the sling, hooks it up to the front tow, then keeps retreating. When the truck is loaded and ready to leave the shovel face, a motor on one side of the sling starts to pull, giving the truck the added rimpull. As I understand, the system on an aircraft carrier automatically disengages when the plane gets to a certain point; the same could be done for the haul trucks. This may congest the shovel area? The system may not have the portability to follow the face? It might be worth examining though.

 

[KRSServices]

I aassume that you are using fairly large rock trucks, and not wagons, although in a couple of instaces you referred to "tractors".



KRS Services

http://www.krs-services.com

 

[mabn]

KRSServices,

We are examining the possibility of increasing the payload of the trucks by having a tractor (dozer) pull the trucks at the front. We are examining large scale machines, but I hope the physical principles remain the same regardless if you were using a donkey and a cart.

 

[KRSServices]

Donkey and cart??

By wagons, I was referring to 25 or 30 yard end dump or belly dump "wagons". These are pulled by "tractors" - 500 hp or larger, highway trucks. Just a clarification of terms.

KRS Services

http://www.krs-services.com

 

[mabn]

Thanks for the clarification KRS. Just used donkey and cart for illusrative purposes.... which isn't all that illustrative when I think about it... unless the cart had a small engine in it and the donkey was just helping the cart through some rough stuff.

 

[DTGT2002]

Are these Over the Road trucks and trailers, or off road use only?

If the tires are a highway tire, they may not have the traction required to do the job.

If the vehicles are not intended for highway travel, or limited highway travel, a purpose built tire might get you moving with increased load.

Additionally, higher floatation tires - ie wider tires can be fitted to some vehicles to allow a larger bearing area and reduce the sinking into the soft ground.

Good luck,

Daniel

 

[Executnr]

Mabn
Have you solved your problem? If not, let me know, I have a few ideas.

Wayne

http://www.ksdesignllc.com

 

[mabn]

DTGT2002,

These are off-highway trucks. We have the widest tires we can fit on them as technology allows.

Executnr,

We've developed a solution, but your input would be helpful and appreciated.

I had setup the following relationship:

Usable Pull > Rimpull > Effective Resistance

Usable Pull being the weight on the drive axle * the coefficient of traction.
and
Effective Resistance being the GVW * the total resistance as a percentage (% grade + % rolling resistance).

Using the fact that we know what payload we are limited to right now, I can solve for an 'estimate' of total resistance and coefficient of traction. Or at least come to a range of those numbers that I can feel comfortable with.

I then added in the weight, rimpull and an estimated coefficient of traction for the dozer.

The part I am unclear about is whether or not you can directly add the dozer rimpull to the truck rimpull... which I did.

Based on the relationship originally setup, there are two conditions that are setup:

Rimpull > Effective resistance
The rimpull is at a level that it should be able to pull x amount of weight, and susequently payload.

Usable Pull > Rimpull
Usable pull is based on the payload and must be at a level greater than the rimpull.

So when I solved, I solved for two seperate payloads. Payload must be greater than x to have the entire rimpull apply, and payload must be less than y to overcome resistance. If max. payload for the truck is in that range we are super-green.... which it ended up being.

Again, though, the place where I am unclear is whether adding the rimpull of the truck directly to the rimpull of the dozer was the right thing to do or not.