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Posted: 11/11/2015

Interpreting the Specs to Find the Right Compact Excavator

Bobcat compact excavator (mini excavator) arm configurations.

When it comes to compact excavators, you’ll find that nearly all of the key specifications relate to the machine’s size, power or performance. Depending on the manufacturer, you’ll find a range of sizes, engine horsepower ratings, dig depths, dump heights and arm and bucket breakout forces. So let’s dissect them one by one.

Size
In order to get the most out of your compact excavator, you have to make sure it can access your jobsites and then have the capability to do the work once it's there.

  • First, you’ll want to look at the dimensions of the compact excavator, meaning its width, height and weight. For example, the Bobcat compact excavator line ranges from the 418, weighing 2,593 pounds at 28 inches wide (with undercarriage retracted) to the E85, weighing 18,960 pounds at 91 inches wide. The size of machine you need should be determined by the application, environment and tasks you anticipate.
  • The tail swing option you choose will also affect your compact excavator’s width and weight. Generally, a Zero Tail Swing (ZTS) or zero house swing (ZHS) compact excavator of a given size will be wider and heavier than its conventional-tail-swing counterpart. So it’s definitely something to keep in mind when thinking about the areas in which you’ll be working.
  • Some manufacturers also offer compact excavators with retractable undercarriages, which allow you to make the undercarriage narrower to pass through a gate or fence, and then to expand the undercarriage when actually working

Working Range Specifications

Dig depth

This specification is defined by the maximum depth that the excavator is able to dig on flat, level ground. It is important to realize that an excavator is only capable of reaching its maximum dig depth for a small portion of the machine’s workgroup path. Therefore, you may want to select a machine that has 20 percent more dig depth than your routine requirements. By doing this, you will significantly minimize machine repositioning and enhance productivity.

Reach
An excavator’s reach is measured from its center rotational axis to the tip of the standard bucket tooth at ground level. This is an important specification, especially if you’ll be digging trenches, because greater reach means less repositioning and more productivity. If you want more reach and dig depth, you might want to consider excavator models with long-arm and extendable-arm options.

Dump height

The clearance that your compact excavator will need to dump the material from its bucket is called dump height. It’s an important selection criterion, especially if your excavator will be frequently used for truck loading. I’s important to note that dig depth, reach and dump height are affected by the excavator’s arm options — standard arm, long arm or extendible arm.

  • The long-arm option has a longer dipper arm that reaches farther from one spot to do more work without repositioning the machine. The extra reach of a long arm, combined with a higher dump height, also makes loading trucks faster and easier.
  • An extendable-arm option is a telescoping arm that gives you the best of both worlds — power in the retracted position, plus the ability to extend the machine’s arm for more dig depth and reach when you need it.

Performance Specifications

Engine horsepower
The reason you wouldn’t purchase a truck with the smallest-horsepower engine is the same reason you should demand more horsepower in a compact excavator. By having more engine horsepower, your excavator is able to generate more hydraulic horsepower when it counts — under tough digging conditions. When looking at horsepower specs, make sure you compare apples to apples, as some manufacturers publish net horsepower, while others publish gross horsepower. Also consider the availability of a turbocharged engine if working at higher altitudes, as they sustain usable horsepower better than a naturally aspirated engine.

Hydraulic Performance

Arm and bucket breakout forces
Two of the more commonly published and compared compact excavator specifications that you may be familiar with are arm breakout force and bucket breakout force.

  • Arm breakout force is the excavator’s ability to produce a “pulling force” using the arm hydraulic circuit. You might also have heard this referred to as arm, dipper and crowd force.
  • Bucket breakout force is the excavator’s ability to produce a “prying force” using the bucket hydraulic circuit.

It’s also important to note that published arm and bucket forces are theoretical (i.e., they are calculated by each individual manufacturer).

Cycle times and multi-functioning hydraulics
So what are cycle times? It’s the amount of time, measured in seconds, that a particular function takes to be cycled (i.e., boom up).

Remember, faster cycle times mean you’ll be getting the most productivity out of your compact excavator — whether you’re digging, trenching, grading or grappling. Some excavators provide multifunctioning hydraulics that enable you to use up to four functions at the same time (e.g., curl bucket, raise boom, extend arm and slew). You would typically use all of these functions when digging a hole or loading a truck.

As mentioned above, if your excavator has more engine horsepower, it’s going to provide more flow at higher pressures — which translates into faster cycle times under load.

Auxiliary hydraulics
Auxiliary hydraulics is a dedicated source of hydraulic oil intended to provide oil flow for specific attachments. An excavator’s auxiliary hydraulic flow rates and pressures are a key determinant of its attachment capacity and capability.

Rated lift capacity
Because excavators are frequently used for lifting objects on jobsites, you need to know the working limitations of the machine. The rated lift capacity often dictates what attachments are approved for the machine. It is important that an excavator’s rated lift capacity be adequate to handle the attachments you intend to use.

All Bobcat excavators are equipped with a rated lift capacity decal, illustrating three general lifting positions:

  1. Boom over the blade, with the blade down
  2. Boom over the blade, with the blade up
  3. Boom over the side, with the blade up

In addition to these lifting positions, you should also be aware of the anticipated lift point radius and lift point height. The lift point radius is measured horizontally from the excavator’s center swivel point to the bucket pivot pin. The lift point height is measured vertically from ground level to the excavator’s bucket pivot pin.

So what are the questions you should ask when trying to estimate the lift capabilities of a particular excavator?

First, determine which position the excavator will be in when attempting to lift the load and whether the machine will be repositioned while lifting the load. For example, even though the excavator is able to lift an object when lifting over the blade, that load may exceed the excavator’s lift capacity if the operator slews the load over the side of the machine. So if you’re anticipating multiple lifting positions, you might need to establish the machine’s worst-case-scenario lift capacity, meaning over the side at maximum reach and at ground level. In nearly all cases, you’ll achieve the greatest lift capacity when the load is lifted over the blade, with blade down and the load is nearest the machine.

When talking about excavator lift capacity, it’s also important that you understand the difference between hydraulic limits, which occur when there’s not enough hydraulic force to lift the load, and stability limits, which occur when the hydraulic force makes the excavator unstable.

The key to understanding excavator lift capacity lies in being able to understand the lift chart provided in your machine’s operation and maintenance manual. The lift chart illustrates your excavator’s lifting capabilities under the various machine positions defined above. In order to effectively use your lift chart, you must establish the lift radius, the lift height and the lifting position. It’s also important to note that when comparing excavator brands, you might find that not all manufacturers are comparing apples to apples to determine this specification. For instance, when establishing an excavator’s lift capacity, some manufacturers include the weight of a bucket, while others do not. Bobcat excavator lift capacities are established using their standard quick-tach attachment mounting system and bucket.

Now that you’re armed with the knowledge to interpret excavator specifications, you’ll still want to do one very important thing before making the decision to buy — demo your top choices. Not all compact excavators are created equal. Machines may appear to operate close to the same speed when sitting in a parking lot, but they might have completely different characteristics when placed under load. That’s why you shouldn’t rely on simple dry runs to determine your purchase.

Try your targeted excavators in real-life situations, such as loading trucks or trenching. You may find that some machines outperform others in production and speed by as much as 30 percent — simply due to the balance between the hydraulic system and engine horsepower.

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