Most pull scrapers require a special hitch on the tractor. The weight of the scraper places too much stress on a standard ag tractor drawbar.
"This is the reason Hoelscher scrapers utilize the lower arms of the three-point hitch," says Hoelscher. "We are able to transfer sufficient weight to the tractor without needing to modify the drawbar, as is necessary with most scrapers that don't have dolly wheels. Our hitch provides for three axis of rotation around a common point. The design prevents excessive wear on the pivot points."
Every manufacturer offers horsepower recommendations for their specific scrapers.
"It varies a lot," says Miskin. Some scrapers are easier to load than others. "For instance, we have one competitor that makes a 17-yd. scraper that is going to take a 400-hp tractor to load it. Miskin makes a 17-yd. scraper that can be loaded with about a 250-hp tractor in the same time and distance."
The design of the scraper has a lot to do with how easy it is to load. For example, an ejector scraper can require more horsepower to load than a dump scraper. "It is just a different shape on the bowl and its design," says Miskin. Soil conditions also impact the horsepower needed.
"On sizing the tractor, it is really the weight and traction of the tractor that is more important than the horsepower for loading," says Miskin. "A 300-hp tractor that weighs 30,000 lbs. will not pull as well as a 300-hp tractor that weighs 40,000 lbs. For transporting, especially up a steep grade, then the horsepower comes into play more. The most popular configurations are a 400- to 450-hp tractor that weighs 45,000 to 55,000 lbs. pulling double or triple 19-yd. scrapers. This is a good match for most jobs."
Adding weights to the tractor to help it load the scraper is not necessarily the right approach. "There are some economies making the tractor barely heavy enough, so it is not too heavy," says Miskin. "If you buy the largest tractor and weigh it down with optional weights, you are going to burn a lot of fuel hauling the extra weight around. It is also harder on the tractor drive system. The heavier something is, the shorter the life of the entire unit is going to be."
Several manufacturers have modified ag tractors specifically for commercial scraper work. Case offers its STX tractors in scraper versions. "The tractor is tailored for the scraper market," says Roger Lewno, marketing & training manager - HHP tractors and combines, Case Corp. "Our cab is not only ROPs certified as it is for ag, but we are also FOPs certified. We place the scraper drawbar on the tractor and we place a tow cable so that the tractor can be towed in the event that it does get bogged down." In addition, the tractor is fitted with backup alarms and heavy-duty axles with differential locks.
To protect the transmission from operator abuse, the start-up gears have been programmed at lower settings than the ag tractor equivalent, and it has a kick-out default. "If the tractor gets lugged down below a given rpm, we will throw the tractor in neutral," says Lewno. "It saves the transmission from being abused."
The tractors are also ballasted for scrapers. "Let's say the scraper itself is an 18,000-lb. unit," says Lewno. "Approximately a third of that weight is on the drawbar. You need to offset that with weight up front with an empty scraper. If you take 17 yds. of material at roughly 2,000 lbs. per yd., you have a third of that weight also on the back of the tractor."
Laudick explains, "Scraper tractors are normally ballasted somewhat lighter than agricultural tractors due to high spike loads during the scraper loading process, as well as a weight transfer from the scraper tractor. As a scraper is loaded, then raised to the transport position, considerable weight is transferred from the tongue of the scraper to the rear of the tractor pulling it, thus reducing the need for weight on the tractor. For these reasons, typical scraper tractor ballast levels will run 10% to 15% less than a comparable agricultural tractor."