Hausner adds, "We recognize our ejector scrapers as more construction scrapers than our pivot-dump scrapers because they are heavier. They will get the mud out easier; they will push it out over the blade."
Dump-style scrapers have faster cycle times. "They have the advantage that they dump fast, but they usually just dump in a rough pile," says Miskin. "They don't really precision finish while they unload. A lot of them have a reputation of being difficult to unload because they don't dump very steep." This can create a challenge in difficult soils.
"On the other hand, carry-all scrapers dump the material and are less money than their ejector brothers," says Stickler. "On a dollar to dollar basis, the carry-alls are about 10% to 15% less money." Carry-alls also have a slight ground pressure advantage because of their lower weight.
The tip-out ejector combines traits of both. "It will do everything that a push-off ejector will do as far as unloading on grade and precision finishing, but it has the speed of the dump style," says Miskin. "With a 30-gpm flow, the tip-out ejector will unload in 5 seconds. It is going to take 24 seconds for an ejector in the same position."
Another consideration is tire size. "Larger scraper tires provide better flotation and less rolling resistance for more efficient use of horsepower when transporting, especially in soft conditions," says Miskin.
For applications where hitch weight must be kept to a minimum, there are dolly wheel scrapers.
"The advantage of the dolly wheel unit is there is very little tongue weight," says Miskin. "If your tractor is not able to handle the tongue weight, then you can use the dolly wheel unit and still get the work done." An example is a crawler dozer with metal tracks. "Most of them can't handle very much extra tongue weight. Hauling around that extra weight, depending upon the dozer, can really shorten the life." The dolly wheels protect the dozer from damage.
The downside of the dolly wheel scrapers includes decreased maneuverability. Darrel Hoelscher, Hoelscher Inc., also notes that the dolly wheels add rolling resistance.
"If the ground is real soft, the dolly scraper will have a tendency to get stuck a little more often than a [conventional] pull scraper because those tires are carrying a third of the weight of the scraper," says Hausner.
Strike a balance
"Balance of the tractor/scraper combination is the main factor in scraper productivity," says Hoelscher. "Sufficient speed is necessary to move material to the back of the bowl."
Maintaining traction is critical to maintaining the necessary speed. "It is important that the scraper transfers weight to the tractor to improve traction as the load is increasing," says Hoelscher. "Ideally, the tractor will run out of power at the same time that it loses traction."
A lot of emphasis must be placed on hitch design. "Hitch design plays a crucial role in the overall performance of a tractor-scraper combination," says Robin Pett, Bell Equipment. "Most importantly, the hitch design determines weight transfer, turning radius and tractor front to rear weight ratio. The height of the hitch can be an important factor, depending on the conditions of the jobsite."
John Deere manufactures both tractors and scrapers. "Our two factories work closely to perfect the performance of the two machines hooked together," says Greg Laudick, Deere division marketing manager - scraper tractors, John Deere Construction and Forestry Co. "We jointly designed and just introduced the AutoLoad system. With one touch of one button, it loads the scrapers. It maximizes the available horsepower and traction of the tractor to efficiently load the scrapers.
"We also worked together on hitch design, which was a critical area," Laudick continues. "Shorter tongues on scrapers transfer more of the weight to the rear of the tractor. This can induce higher than desired loads, affecting long-term reliability of the tractor. Conversely, the hitch design on the rear of the tractor also has an impact on the weight transfer aspects. If the 'pivot point' is designed and placed too far from the center line of the tractor's axles, that can also affect the long-term reliability."