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Credit: Photo: Thomas Equipment

With offset digging, compact excavators are able to keep their tracks parallel to the cut and stay off soft ground.
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Credit: Photo: Bobcat.

The ability to dig in the offset position is one of the compact excavator's great strengths

Most public works departments don't do extensive earthwork prior to placing concrete. There are, however, many situations where you need to do some digging, and the invention of the rubber-tracked compact (or mini) excavator has been a real boon for those times.

Compact excavators generally are defined as those with operating weights less than 6 tons, and they go as low as 0.9 tons. If you don't have a compact excavator and are thinking of buying, leasing, or renting one, there are a few advantages over the traditional loader backhoe that you should consider.

One big advantage with the compact excavators is the ability to dig in the offset position (see glossary, location). With the boom slewed one way and the carriage rotated the other, offset digging allows the operator to excavate directly adjacent to an existing structure. It also allows the tracks of the excavator to remain parallel to the trench for efficient repositioning. The limitation with compact excavators, however, is that you have a limited digging depth and force.

PURCHASING CONSIDERATIONS

If you are thinking about buying a compact excavator, the first thing to consider is whether you really need one, or you just want one because they are cool. If you are going to buy one, the first decision is size (digging depth, digging force, dump height, reach), which directly relates to the applications you have now and expect in the future. Get a machine that is big enough to do the job, but don't go too big.

There are many manufacturers making these machines today. Some factors to consider when comparing manufacturers and models are:

  • Cycle time—related to the cycle time on the cylinders that control slew speed and boom speed; what kind of productivity do you need?
  • Auxiliary hydraulic flow rates and pressures—relevant to attachment capacity and compatibility; make sure the machine has the flow and pressure you need
  • Travel speed—important on larger jobsites
  • Operator comfort and visibility—related to cab size and design
  • Size/configuration of controls
  • Accessibility of engine and hydraulic components for maintenance—plus overall ease of maintenance.
  • Zero swing (sometimes called zero tail swing or zero house swing) is the hottest thing in the compact excavator market today—but it adds some cost. Operator comfort can be a big issue, since some makes are not built with the larger American body in mind.

    So what does a compact excavator cost? The smallest machines start a little less than $20,000. At the top end of this category (12 tons), prices range from $60,000 to $100,000.

    — William D. Palmer Jr.

    Glossary of key terms

    Arm—Also referred to as the dipper, it is the structure that connects the excavator's attachment to the boom.

    Arm force—The excavator's ability to produce a pulling force, using the arm hydraulic force. This is also referred to as “dipper force” or “crowd force.”

    Attachment—Any device coupled to the workgroup to complete a prescribed task. The most common attachment for an excavator is a trenching bucket. Other attachments may include grading buckets, hydraulic breakers, plate compactors, augers, or rippers.

    Auxiliary hydraulics—A dedicated source of pressurized hydraulic oil, intended to provide oil flow for specific attachments. The excavator's pump system is the source of the oil, and it is routed to the attachment via tubelines and hoses on the workgroup.

    Backfill blade—A hydraulically activated blade attached to the undercarriage and used for grading and backfilling. It also can be used as a stabilizer during digging and can be used to increase dump height and digging depth depending on its position in relation to the workgroup.

    Boom—The primary component of the work-group that is attached to the house structure via the swing frame. It supports the arm and attachment.

    Bucket breakout force—The excavator's ability to produce a “prying force” using the bucket hydraulic circuit.

    Cab—House.

    Compact excavator—Also called “mini” excavators. Compact excavators generally include those with operating weights of 12,000 pounds (6 tons) or less and digging depths of 14 feet or less.

    Control patterns (International Organization for Standardization, ISO, or Standard)—The operating pattern of an excavator's joysticks. There are two predominant joystick patterns on compact excavators. When operating in the ISO pattern, the right-hand joystick controls the boom up/down function and the left-hand joystick controls the arm in/out function. With the “standard” pattern, these two functions are reversed.

    Counterweight—A weight added to the rear of the excavator's house structure in an effort to improve its lifting characteristics. Counterweights also are added to the machine to accommodate variations in arm configurations such as long-arm options or extendable-arm options.

    Cycle time—Refers to the amount of time, usually in seconds, that a particular function takes to complete (for example, to lift the boom).

    Dig depth—The maximum depth that the excavator is able to dig on flat, level ground.

    Dipper—Arm.

    Dump height—The clearance required for an excavator to dump the spoil from its bucket.

    Flotation—The machine's ability to traverse soils or surfaces that have low load-bearing capacity.

    Gear pump system—An oil supply mechanism that produces oil flow from rotating gear assemblies within a pump housing. A gear pump system has a fixed displacement and requires a change in pump shaft speed to affect pump volume.

    Ground pressure—The force that the machine exerts on the surface, measured in pounds per square inch (psi). Lighter excavators do not necessarily have lower ground pressures, depending on the size of the track footprint.

    House—Also sometimes called cab, includes the operator compartment, the engine compartment, and the hydraulic pumps. The house structure is attached to the undercarriage with a swing bearing that allows it to rotate or slew, typically 360 degrees.

    Independent boom swing—The boom swings independently of the rotation of the house. The primary purpose of boom swing is for offset digging around obstacles or along foundations, walls, and forms. A secondary use is cycling in areas too narrow for cab rotation. Independent boom swing is one of the major advantages of a compact excavator over other excavation equipment.

    Offset digging—The practice of excavating with the boom offset to either the right or the left. Offset digging is accomplished by using the independent boom swing capability of the compact excavator.

    Piston pump system—An oil supply mechanism that produces oil flow from a rotating assembly of small pistons within a pump housing. Unlike the gear pump system, the piston pump system has the capability to vary oil flow independent of pump shaft speed.

    Reach at ground level—Measured from the excavator's center rotational axis to the tip of the standard bucket, at ground level.

    Slew—To rotate the excavator's house assembly. Unlike a conventional backhoe, the operator can “slew” the entire house and workgroup upon the undercarriage for spoil placement. This introduces minimal fatigue to the operator as his body is traveling with the work-group for maximum visibility and minimal body movement. Compact excavators can slew a full 360 degrees.

    Tail swing—Refers to the rear overhang of the excavator as it rotates upon the undercarriage. It is measured from the central rotational axis to the furthest rear point of the machine.

    TOPS/ROPS—TOPS is an acronym for Tip Over Protective Structure. ROPS is an acronym for Roll Over Protective Structure. A TOPS or ROPS rating defines the protection afforded an operator in the event the machine would tip or roll over. The structural integrity of an operator's compartment must meet standards prescribed by the ISO or Society of Automotive Engineers (SAE) to be certified as TOPS or ROPS.

    Tractor/loader/backhoe (TLB)—Main competitor of the compact excavator, the TLB is limited to a maximum of 180-degree work-group movement, unlike a compact excavator that can slew 360 degrees. The TLB typically features fold-down stabilizers (outriggers) that often restrict its use in confined areas.

    Undercarriage—Supports the house structure and includes the tracks, drive sprockets, rollers, and idlers.

    Workgroup—The work-group consists of the boom, the dipper, and any attachment.

    Zero house swing (ZHS)—Sometimes incorrectly called zero tail swing, this means that the house and counterweights stay within the machine's width during slewing. This prevents accidentally damaging trees, buildings, and the machine. ZHS also permits easier spoil placement in tight quarters.

    360-degree cab rotation—The ability to continuously rotate the house 360 degrees. Because of the compact excavator's 360-degree rotating house, an operator can dig on one side, then rotate and unload the dirt into a truck or onto a pile several yards from the hole. This allows for enhanced spoil placement, superior visibility, and minimal operator fatigue.

    This glossary was adapted from Bobcat Co., West Fargo, N.D.