Excavator Stability and Lift capacity rating:
By: B. Ravindra
Machine stability is one of the most important measures of a well designed machine and lift capacity is its consequence. Stability is the final outcome of how total weight of machine is distributed over an excavator. Since undercarriage of an excavator is never a square and tilt lines distances from slew center differ between front and side, lift capacity is measured both over front and over side. Lift capacity rating over side is often lower and so is a considered a better measure of functional stability. Lift capacity chart guides an operator on safe lifting loads at a various distances from the slew center. Since safety is involved, ISO 10567 lays down strict guidelines on how and what values are to be reported and what are the safety factors to consider. ISO 10567 uses balance point condition to record the tipping value. The standard also details methods for testing and calculating the tipping loads. Below is an extract from a manufacturers catalog:
When loads are lifted close to the machine, the maximum lift capacity is no longer limited by machine tipping limit but by the cylinder pressures generated by the load. When the cylinder pressure exceeds the hydraulic cylinder relief setting the load is not sustainable and constitutes the hydraulic limit. Both tipping and hydraulic limits are measured simultaneously and lower of these values are considered for rating purposes. For reporting, rated lift capacity values, test values are reduced by 25% if machine hit the tipping limit first and by 13% if hydraulic limit is lower than tipping limit. The above figure uses SAE J1097 which was superseded by ISO 10567 in 2008. Manufacturers distinguish limiting condition used for rating chart. In the above extract hydraulic limit values are distinguished by * ahead of the value.
As can be expected, excavators with blade, have front lift capacity predominantly hydraulic limited. Over side, regardless of machine being with or without blade, the values are mostly tipping limited. Also, lift capacity changes from hydraulic limit to stability/tipping limit with increasing lift radius. While it is generally true that by increasing the counterweight one can make machine more stable, there are limits to such a solution. A machine with poor stability arising from grossly under-dimensioned undercarriage cannot be easily corrected by increasing the counterweight alone. Also, increasing counterweight has to take into account the fact that, while machine stability changes linearly with distance (to CG of counterweight), rotational inertia changes by square of the same distance. Inertia impacts machine dynamic performance and efficiency of slew cycle. Given the importance of stability as a key measure for excavator performance there are instances where, reported values are either inflated or values are published without sufficient testing. A customer has no way of telling if manufacturer published data is correct without actually repeating the test. We detail a method below that comes close to providing an easy way to check if manufacturer published data are indeed correct. The method uses the difference between front and side tipping capacities together with published track dimensions to estimate the weight of the machine. This calculated operating weight is checked against the reported operating weight. It is expected that these two values should be within 5% of each other. A sample calculation at the end of this document for 20ton excavators from different manufacturers shows that this procedure is indeed useful. Differences between front (over FTL) and side lift (over STL) capacities are primarily due: a. FTL (front tilt line)and STL (side tilt line) not being a perfect square b. CG of undercarriage not being collinear to the slew axis Considering undercarriage CG to be collinear to slew axis, is not only a reasonable assumption but paves way to define the lift capacities purely on STL and FTL differences alone as detailed below. Scope: The procedure described below checks pair-wise accuracy of “stability limited” rating values published in Lift Capacity Rating Charts of manufacturers per ISO 10567 for Hydraulic Crawler Excavators. Limitations of the procedure: 1. Pair-wise values mean = a pair of (Rating over front, Rating over side) values for same Load Point Radius (LPR). This procedure does not calculate individual lift capacity values. This means, when published values do not tally with the calculated check value,
it is not possible to deduce using this procedure which of the ratings are wrong – either one of the pair or both the rating values could be wrong. 2. For hydraulic limited lift capacity rating, formula below defines only an upper limit for pair-wise consistency. 3. The procedure is limited to Crawler hydraulic excavators without dozer blade. In machines equipped with dozer blade most of the ratings are hydraulic limited and this procedure cannot be applied directly. Key Assumptions: CG of Undercarriage is assumed to be collinear with the Axis of rotation (AR). This procedure is still valid if the distance between Undercarriage CG and AR is very small compared to the total distance between Rear and Front Tipping lines. Symmetric undercarriage is a reasonable assumption. Since ISO requires that if the undercarriage is unsymmetrical, the machine should be positioned in the least stable position for tipping limit calculations; our assumption of symmetry should give a conservative value. Variable names (Refer fig below) Front and Side tipping conditions for same LPR) are super-imposed. The hatched areas are tracks in side tipping condition. 1. 2. 3. 4. 5.
Wf = (Front Lift capacity Rating / 0.75) (known) Ws = (Side Lift capacity rating / 0.75) (known) LPR = Load Point Radius (front) = Load Point radius (side) (known) a = Distance between FTL and RTL b = Distance between ST
Calculating ‘a’: a is the distance between FTL (front tipping line) and RTL (rear tipping line). For track type undercarriage, per ISO 10567, the line joining the centers of support idlers is FTL and center line of sprockets is RTL. Since we are assuming the undercarriage to be symmetrical, FTL and RTL are equidistant from the slew center and a = Distance between tumblers (distance between sprocket and idler centers) Calculating ‘b’: ISO 10567 for track type undercarriages gives the side tipping lines (TL) for balance point calculations as below:
For right-extreme track scheme, assuming ‘b’ to be equal to Track Gauge (distance between left and right track center lines) wouldn’t give very wrong results. For other two track chain configurations, use the table below for calculating ‘b’. b = Track Gauge + (A or B)
Weight 6 - 10 tons 11 - 14 tons 15 - 19 tons 20 - 22 tons 22 - 25 tons 26 - 30 tons 31 - 50 tons 51 - 75 tons
A (mm) 102 143 171 171 185 205 247 274
B (mm) 132 179 215 219 239 257 307 356
Now with a and b calculated Wf, Ws and LPR from published lift capacity data; one can calculate the following values: m = Wf. (LPR - a/2) where Wf = (Front published rating / 0.75) n = Ws. (LPR - b/2) where Ws = (Side published rating / 0.75) k = [(a – b)/2] Wc = [(m – n)/k] Now check if Wc ≤ Wp where Wc is calculated Operating weight and Wp is Published Operating Weight of machine. If Wc and Wp are within ±5% of each other as set out by ISO standard, then you have a valid pair-wise accurate stability rating. Let’s look at a sample of manufacturer’s stability data and see how they fare with our accuracy check. While data is from real machines, all the model names have been changed:
Excavator Models -> Wp, Operating weight Published Lift capacity @ Front Lift capacity @ Side or over 360° LPR, Load Point Radius a/2 Track gauge A b/2 Wc, Operating Weight Calculated Variation
kg kg kg m m m m m kg
C203LD H200LZ D225LS K210LS H210LY V210LV 21550.0 20400.0 21500.0 21990.0 22650.0 22000.0 6466.7 6946.7 6280.0 6533.3 6933.3 7200.0 3866.7 4240.0 3906.7 4066.7 3920.0 4533.3 7.5 7.0 8.0 7.5 7.5 7.5 1.8 1.8 1.8 1.8 1.825 1.83 2.4 2.4 2.4 2.4 2.39 2.39 0.2 0.2 0.2 0.2 0.185 0.185 1.3 1.3 1.3 1.3 1.2875 1.2875 22985.0 21225.8 23143.6 21281.5 27895.2 23337.6 6.2% 3.9% 7.1% -3.3% 18.8% 5.7%
For other pairs of data that are hydraulic limit rating values, including those that have one of the pair stability limited, Wc should be definitely less than Wp. If it is not then, one or both the published lift capacity values are wrong. Test engineers measuring lift capacity can use this equation to estimate side tipping limit if front is already tested accurately.
