Sizing Up Skid-Steer Loaders

The primary use of a skid-steer loader is as a power source," says Kelly Moore, skid steer and track loader product manager, Gehl Co. Versatility to run a wide array of attachments means performance can be measured using several different criteria.

"Compare all of the specifications before making a decision," says Moore. This includes the specifications of larger and smaller models from the same manufacturer, as well as competitive units. Compare dump heights, bucket rollback angles, hydraulic pressures, hydraulic flows, engine power, breakout force, etc.

The optimum loader for your fleet is based on several operating parameters. "The loader's weight, horsepower, hydraulic flow and hydraulic pressure will play a role in how well the attachment performs," says Mike Fitzgerald, Bobcat product loader specialist.

Match hydraulic flows and pressures

Hydraulically powered attachments require a specific level of performance from the carrier's hydraulic system. "An attachment usually has a range of flows and pressures," says Moore. Suitable carriers must be able to provide flow and pressure that fall within that range.

Hydraulic horsepower can also be a useful comparison measure to determine the capability of a given unit. It is calculated by multiplying the pressure (psi) times the flow (gpm), then dividing by 1,714. "If hydraulic horsepower is not adequate, performance will be poor, as well as detrimental to the skid-steer loader," says Jim Hughes, product marketing manager, Case Construction Equipment.

In addition to losing attachment productivity, using a skid steer that can't provide the optimum hydraulic system requirements places greater stress on the unit. "Operating a loader at the maximum flow and pressure for long periods of time can result in excessive heat," says Moore.

Hydraulic systems are designed with relief valves to protect them from such abuse. However, Hughes, notes, "If you are constantly going over relief, you will be creating heat, which will ultimately damage the hydraulic system and the power train."

Gregg Zupancic, product marketing manager, John Deere Construction adds, "If the machine has more flow and the attachment does not have the proper flow bypass, heat could be created and potentially cause motor and seal failures. If the machine has less flow than the attachment requires, then the attachment may not run at optimal performance and it can potentially cause unnecessary failures to mechanical or structural parts of the skid steer and attachment. A good example would be a cold planer. If the drum is not spinning at optimal speed, instead of planing the asphalt, it would tend to chuck the surface and cause unnecessary machine bounce or attachment vibration."

Jorge DeHoyos, brand marketing manager for New Holland Construction, adds, "Attachments typically list minimum and maximum flows and pressures. Using the wrong attachment can cause physical damage and premature wear on both the skid steer and attachment. Using a cold planer with a lower than required flow capacity, for example, will lead to premature breakage and wear on the cutting tips. Using an auger that requires more flow than provided will overheat both the attachment and skid steer, since the oil runs through both."

But there are measures other than flow and pressure that can affect hydraulic performance. "Another critical item to check is back pressure of the skid-steer loader," says Hughes. "High back pressure robs performance because the machine is working harder to return the oil to the tank."

The role of rated operating capacity

While hydraulic power plays an important role, lifting capacity must also be considered when determining attachment compatibility. "For example, a backhoe attachment can have a considerable amount of weight, but does not require a large amount of hydraulic flow," says Zupancic. "In addition to making sure that the skid steer is compatible with the attachment flow, the customer also needs to be able to safely carry the attachment around the jobsite on all four wheels."

Rated operating capacity plays an even larger role with attachments designed for lifting. "For example, Bobcat manufactures pallet fork attachments for both small-frame and large-frame skid-steer loaders (up to 3,000-lb. rated operating capacity)," says Fitzgerald. "Obviously, the large-frame skid-steer loader with heavy-duty pallet fork can lift more than the small- or medium-frame skid-steer loaders."

But being able to lift the load is not the only consideration. "Rated operating capacity is not only critical for the skid-steer loader to remain safe and stable while lifting and carrying the attachment, but also to provide adequate down pressure on attachments like cold planers, trenchers, tillers, etc.," says Hughes.

The rated operating capacity can often be slightly increased by using an optional factory counterweight package. "For customers at the upper end of the rated operating capacity, one model counterweight can add an additional 150 lbs. of capacity," says Moore, discussing Gehl's skid-steer models. "In addition, it serves as a bumper and hitch. The counterweight on our two larger E Series loaders increases the rated operating capacities by 300 lbs."

Similarly, Bobcat offers factory-approved counterweight packages through its dealers. "Loader owners can add counterweights — available from their dealership — to the machine for an increase in the loader's rated operating capacity," says Fitzgerald. "Even so, the loader owner should still closely follow the manufacturer's recommended limits for specific attachments and carriers."

Attaching weights to the front of the machine can help with the performance of certain attachments. "As long as the counter balance is within the recommendation limits of the manufacturer, you can generally put as much weight on the front of the machine as needed because the limiting factors will be the hydraulic system breakout forces and the tipping load of the machine," says Zupancic. "Constant heavier loads will accelerate wear over the life of the machine. For example, constantly lifting 4,000 lbs. versus lifting 100 lbs. will increase wear on the pivot points of the boom linkage over time."

Wheelbase further impacts capacity

The wheelbase also affects the rated operating capacity. "For example, Bobcat has three wheelbase options to choose from," says Fitzgerald. "The small-frame loaders' wheelbase is approximately 36 in.; the medium-frame loaders are 40 in.; while the large-frame loaders are 48 in. The large-frame skid-steer loaders' wheelbase allows for higher tipping loads and rated operating capacities."

In addition, the extended wheelbases enhance stability. "By extending the wheelbase on New Holland skid steers, we can better distribute the weight over a desired footprint and use the laws of physics to our advantage," says DeHoyos. This allows the skid steers to lift more with less vehicle weight. "The wider wheelbase gives the additional benefit of stability and gives the skid-steer operator more confidence."

Of course, there is more to rated operating capacity than the stance of the machine. "Extended wheelbases contribute to additional lift capacity, but there are other factors that also contribute," says Zupancic. "For example, total machine weight and the geometry of the boom design can also contribute to increased lifting capacities."

Check compatibility

Not all skid-steer attachments are compatible with every make of skid steer. One example is the recent introduction of undercarriage kits that convert skid steers to compact track loaders. Before installing a tracked undercarriage conversion kit, be sure to check whether the manufacturer of your particular skid steer approves their use so you don't void the machine warranty.

Certain attachments also place greater stress on the machine. "Hammers and cold planers typically place the greatest demand on a skid-steer loader," says Hughes. "The hammer is constantly pounding and all of that energy is transferred to the skid-steer loader. A cold planer, as it grinds up pavement, has similar characteristics. That's why those attachments are recommended for use on higher-capacity skid steers, which are engineered to withstand the abuse the attachments dish out through their everyday use."

Keep it cool

Heat is the enemy of any hydraulic system. "Any attachment that creates excessive heat can be detrimental to hydraulic components of a skid steer," says Zupancic. "For any attachment operating in abnormal or abrasive conditions, it is critical that the customer and the dealer are communicating with the manufacturer of both the skid steer and the attachment to make sure the units will perform and be reliable. In some cases, additional cooling or a custom debris package may be needed to run an extreme attachment."

Some manufacturers offer optional oil cooling packages to address this issue."John Deere machines provide industry leading cooling capabilities for 95 percent of attachments. However, there is a custom cooling package, which can be installed at the dealer level in conjunction with John Deere Product Support, that will provide extreme attachment application cooling," says Zupancic.

Other machines come equipped from the factory for handling extreme conditions. For example, Gehl models come with oil cooling packages already installed.

It's important to keep in mind that skid-steer loaders are called on to perform a wide range of applications, and each machine provides different levels of performance in several key areas. Identify the characteristics that are important to your rental customers so you can determine which models are best matched to their specific applications.

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