Tipping Stability, Lifting and Slopes

In this article we will take a look at the tipping stability of mini excavators to help you have a better understanding of the physics at play so you can operate your machine more safely.

The tipping point happens when the forces exerted by bucket/tools and load shift the center of gravity (balance point).

The mini excavator is designed to be at an equilibrium and have stability while handling a given load (mass) with the bucket extended at a given range and height from the ground. Those are the operational limits or load lift capacities.

The Center of Gravity (CG) is the point where the excavator’s weight is considered to be concentrated. The CG should be within the center of our slewing ring to reduce stresses on our slew assembly, but in practice we know that's not the case.

On both sides of the CG we have forces that must be at an equilibrium for a machine to be balanced and stable. When the excavator lifts a load, it creates a torque (rotational force) around the pivot point. The stability depends on the sum of moments (torques) around the pivot point.

The moment due to the load must be counterbalanced by the moment due to the weight of the excavator. Mathematically the mini excavator is in static equilibrium when the sum of all forces and the sum of all torques acting on it are zero.

The machine undercarriage and upper structure are a constant force so balancing is in practice controlling the weight of the load, how far the load is extended from the counter mass and how high is the load. To understand this even better let's look at the math. The basic formula of balance on a flat surface is:

This formula is used like this:

W1	The weight of our load. (weight of bucket + weight of tools + load)
D1	The distance of the center of the load to the pivot point.
W2	The weight of the excavator upper structure and undercarriage.
D2	In practice the length of the machine’s main body.
WC	The weight of a counter weight if any is installed.
DC	The distance of the center of the counter weight to the pivot point.

If I have a mini that has a counter force of 700 kg/m then the weight of a load at a given distance can not exceed 700 kg/m force (torque). That means that I can handle a 700 kg load when the bucket is at one meter, 350 kg when it's at 2 meters, etc. (I used metric units to keep the math and numbers simpler.)

The height of the load actually increases the forces on the bucket due to added gravitational force. The new formula looks like this:

When we add slopes things become even more complicated as now we have to account for gravitational forces parallel and perpendicular to the slope. Forces parallel to the slope affects the machine’s tendency to slide or tip over. The slope angle increases the torque of the load side with respect to the machine body.

In practice the balance is never perfect and it is more of a ratio. How much load we can handle without passing our tipping point and not stressing our swivel assembly.

The lifting capacity for bucket operation is defined by ISO 10567. A 1 ton machine sold in the EU must have a rated bucket lifting capacity, here in the US they are not, regardless the lifting capacity of these machines is seldomly published.

The “rated load capacity” (Safe Working Load) is the weight of the load (c) that can be handled at the maximum allowed distance of the load point (b) without tipping the machine. This rated capacity will also give you the maximum height (d) of the lift point (not the load). This rated capacity is actually a balanced ratio around the machine pivot point.

The formulas are:

Lift Height = ( Load Kg x Distance of Bucket ) / Counter Mass

Load Kg = ( Lift Height * Counter Mass ) / Distance of Bucket

Distance of Bucket = ( Lift Height * Counter Mass ) / Load Kg

The manufacturer must be able to provide you with the following information: a chart of Lift capacity ratings (load point heights (d) in relation to load radius/distance (b) ), and a list of parameters for their calculation of stability. The lift chart may be affixed to your machine as a sticker. That is the formal information that you need to operate your machine safely.

The rated load capacity is listed when load is facing front/back and when load is perpendicular to the tracks. The load capacity over the side is almost half of its front load capacity. The rated capacity is always 75% of the tipping load and no more than 87% of the machine’s hydraulic capacity.

There is a lot of confusion about the actual capabilities of a mini excavator when it comes to handling slopes and inclinations. The first culprit regarding the confusion I think is the vague or even misleading information published by vendors. It is common to see 1 ton mini excavators indicating that they can handle 30° of incline. This number will get you in a lot of trouble. We present to you exhibit 1…

Example 1: AGT	Example 2: AGT	Example 3: RIPPA

If you look closely you will notice that for RIPPA and AGT the documentation actually reads 30% of Climbing Ability, notice that is 30% percent and not degrees. In some other brochures and web pages we will also see the 30° ratings for example in some of AGT web pages and literature.

Let's step back and go over some terms. Gradeability or climbing ability is the steepest slope a machine can drive on and the operating angle is the angle a machine can operate without moving.

Gradeability is a percentage of 45°, where 45° is the established max for these types of machines. The term gradeability or climbing ability is generally specified as a percentage of 45°.

You can convert the percentage to degrees using this formula:

Let's use this formula with the common 30% of Climbing Ability supported by 1 ton mini excavators. For 30% we get 16.7° degrees or let's round it to 17°.

There we have it, a common 1 ton mini can handle 17°. Brochures and web pages stating 30° are simply incorrect or hopefully is just a typo. To help visualize what 17° looks like compared to the absurd 30° we made this illustration:

As you can notice, 17° is a very decent incline and we encourage everybody to approach these with extreme caution.

If you have questions about a machine's specifications, you should consider asking the vendor for the machine’s CE certification reports. The CE Certification Reports will include information about many mechanical parameters including gradeability.

Now if you know the run of an incline and its height (rise) (a ramp, embankment, etc) you can compute the angle percentage and see if it's safe for your machine. For example if we have a ramp with a rise of 3 feet and a run (length) of 20 feet, we can compute the slope percentage by dividing the rise by the run. In our example we divide 3 by 20 and we get a slope of 15%.

Let's talk more about gradeability. This number is not so arbitrary, manufacturers have to consider cavitation of fluids like hydraulic oil, engine specifications and the ability to brake.

ISO 10266 establishes the standards for the determination of slope limits for machine fluid systems operation on earth-moving machinery. For example hydraulic fluid is susceptible to cavitation. Cavitation is formed by irregular voids or bubbles in hoses and components. These bubbles explode under pressure creating cavitation wear on the components. Your control valves and pumps are more susceptible to cavitation. The design of the hydraulic tank will also limit the angle at which the fluid can properly flow. Hydraulic tanks have diffusers and a suction line that generally does not reach the tank bottom, thus increasing the chance of air entering your suction line.

Engines also have design limits, for example the Briggs & Stratton oil sump is only rated for 15°. While we haven't found gradeability data for the Kubota D722 engine, machines using this engine from big brands also stay below the 17°, with Kubota own machines keeping the gradeability at 15°.

One issue we have with mini excavators is that given their small size there's very little room to balance the loads. That's why many people call these machines “tippy” or “bouncy”. Changes of speed, working equipment movements, or travel direction create enough inertia to shift the center of gravity and make them bounce.

In a slope the inertia of movements is amplified and it becomes a significant variable at tipping the machine. Slow and steady is crucial when traveling on a slope, especially down hill. This means no changes of direction, no sudden stops, not rotating the upper structure and no sudden adjustments to the working arm.

The lateral tilt angles of these machines are very unforgiven. Traveling parallel to a slope or overcoming a lateral obstacle that tilts the machine in excess of 10° degrees will cause the machine to roll on the side.

Remember when traveling perpendicular to the slope these machines are actually certified for 17° and we tell our clients don't push past 15°. The load and the position of the boom will actually determine what you can get away with safely.

When facing a slope we extend the boom, keep the load close to ground to offset our center of gravity and travel slowly. How far we extend the boom will change the tipping point and balance of the machine. Balancing your center of gravity is important to reduce wear of your swing assembly and motor. Never travel on a slope with the bucket too high/far or to the sides. The more your boom is rotated laterally or lifted the more susceptible the machine is to roll even on flat surfaces. The bucket close to the ground will help in case of emergencies, it may stop the machine from tipping, it can be used as a brake or to stabilize a bouncing machine.

A word of caution with depending on using the bucket/arm/blade to control an unstable or tipping machine. If the risk of traversing a slope depends on your quick reflexes, then you need to step back and reassess the job. The dozer blade and bucket may work well on larger machines to stop an uncontrolled descent or tilt. Nevertheless a small mini has very little thresholds of tippling stability that is not helped by its little weight. On an uneven slope the dozer blade or the bucket can actually induce a more severe rolling.

We need to add that the weight of a small mini puts it in a precarious situation on slopes covered with snow/ice, loose leaves (worst when wet) and wet terrain. These conditions may cause the machine to slip and accelerate which may cause the machine to tip or roll.

There are two camps when it comes to how you drive up/down hill. There is a group that always drives with the blade forward to use it in case of emergencies and for stability. We feel that the small blade in a mini could not be used to control a tipping on a slope and provides too little in terms of stability. The other camp is to keep the sprockets (motors) downhill (the dozer blade in the back of the travel direction). The main argument of the sprockets downhill is to keep the bottom of the track tensioned, reducing the possibility of derailment.

When you transition from flat to a slope these machines bounce and that's when you are at most risk of tipping. You can use your bucket on the ground to control the transition.

Going back on tipping stability we would like to add some pointers on modifying your machine to add counterweights:

Before you consider adding counterweights because your machine feels tippy, consider taking more time to get more comfortable balancing the load by adjusting the placement of the bucket relative to the machine’s center of gravity. This is something that as you get more proficient you should be able to make adjustments as you travel almost without even thinking about it.

Counterweights mods should be removable and adjustable. Your machine was designed at the factory to be balanced at the center of the swing assembly, permanent counterweights will unbalance the machine when the bucket load is below its balance ratio, this will damage your swing assembly and motor. Unless you expect to work with fairly similar loads, having a fixed counterweight will not help much and probably will add more wear to your swing assembly.

In addition to making counterweights adjustables you may go back to the formula and see that the balance is not only counterweight but distance. In our original examples of a 4 to 1 ratio, if we double the placement distance of the counterweight we will reduce the required weight of the counterweight by half. Consider a setup where you can adjust the distance of the counterweight.

If you have comments regarding this article please send us an email to info at mini excav dot com.