Two numbers that belong in the same comparison
Underground loader bucket capacity vs payload is a practical purchasing question: how much space is available for material, and how much material mass is the machine rated to carry? Cubic metres answer the first question. Tonnes answer the second. A larger volume does not automatically mean a higher permissible load, and a load rating does not mean every bucketful will reach that mass.
The distinction matters when two quotations look similar except for bucket size, or when a mine changes the material included in its loading plan. Comparing the numbers separately can hide a poor match. This guide explains how to read them together, using the ZDL307 reference specification and arithmetic examples. It is a procurement comparison, not a loading procedure or a substitute for the applicable operating manual.
What the ZDL307 reference actually records
The supplied ZDL307 technical specification lists a rated load capacity of 7 t at full load and a bucket capacity of 3 m³, described in the source as stacking capacity. These are separate entries. The document does not state that every 3 m³ bucketful weighs 7 t. It also does not establish a universal material density for every installation.
Keep the source wording attached to the volume when comparing offers. A stacking or heaped volume should not be silently compared with another supplier's struck volume. Ask for the bucket measurement basis and the configuration covered by the rating. The ZDL307 product page provides the model context; the underground loader category helps identify the equipment family before a detailed comparison.
The available reference has no verified revision date. Its values are useful for an initial discussion, but the purchase specification should identify the actual bucket and machine configuration. Conflicting turning-radius and other unrelated entries in the source are excluded from this comparison.
Density connects volume to mass
For a stated quantity of loose material, mass equals volume multiplied by loose bulk density. With volume in m³ and density in t/m³, the result is tonnes. The word loose matters: the spaces between fragments form part of the material volume in a bucket. A solid-rock density and a loose bulk density therefore describe different measurement conditions.
For an inquiry, identify what density represents, how it was obtained and whether its moisture condition matches the material being compared. A value copied from a general mineral description may be a poor basis for estimating a bucketful. Different fragmentation and moisture conditions also make an unexplained single density difficult to audit.
Caterpillar's underground bucket guidance connects bucket sizing with material type and density. That supports the general comparison principle, but supplies no density value or bucket approval for ZDL307. The material evidence for a particular mine must come from that project.
Caterpillar: underground bucket selection and material density ↗
Fill factor is another assumption, not extra payload permission
Caterpillar defines fill factor in terms of the share of available volume actually used. For a comparison worksheet, writing estimated loose volume as stated bucket volume multiplied by an assumed fill factor makes this assumption visible. It does not prove that the assumed filling condition will occur on a mine route.
Keep the definition consistent across bids. If one calculation starts with a heaped volume and another starts with a different volume basis, applying the same percentage does not make the estimates equivalent. Record both the reference volume and what the percentage means. Do not borrow fill-factor tables from unrelated materials as measured evidence for a particular operation.
Neither an assumed fill factor nor visible space remaining in a bucket authorizes exceeding the rated payload. Bucket geometry and load mass remain separate constraints. The machine's applicable limits still govern, even if a spreadsheet suggests that more material would physically fit.
A hypothetical calculation using the two reference ratings
Assume, only for arithmetic illustration, that the 3 m³ reference volume is fully represented in the comparison and that loose bulk density is 1.8 t/m³. Estimated material mass is 3 × 1.8 = 5.4 t. This is below the referenced 7 t rated load. It shows why a bucket can reach its stated volume without reaching the mass rating. The density and filling condition are invented for this example; neither is a ZDL307 test result.
Change only the assumed loose bulk density to 2.5 t/m³. The same calculation gives 3 × 2.5 = 7.5 t. That exceeds the referenced 7 t payload and is therefore not an acceptable rated-load assumption. It must not be treated as an instruction to carry that mass. The example exposes an inconsistency in a proposed comparison before it enters a purchase decision.
Dividing the two published figures gives 7 ÷ 3, or approximately 2.33 t/m³. This is simply the density at which those numbers coincide under the assumed full-volume calculation. It is not an approved material density, an operating target, a loading allowance or evidence that other machine limits have been satisfied.
Why the bigger bucket does not settle a productivity comparison
Bucket capacity describes one part of the loading system. A tonnes-per-hour estimate also needs a supported mass-per-load assumption and evidence about completed cycles. Travel distance, receiving arrangements and delays belong in that study. Multiplying a rated payload by an unexplained cycle count produces a number, but gives the buyer little basis for judging whether it is achievable.
The ZDL307 source contains a 16-second entry without a sufficiently clear full-cycle definition. It should not be used here to calculate hourly output. A cycle study needs a defined start and finish, along with the conditions under which it was measured. This is particularly important when a supplier's short hydraulic movement is being compared with a complete load, travel and discharge sequence.
A useful commercial comparison therefore separates documented machine ratings from project estimates. Put estimated production on its own line with its assumptions, rather than presenting it as another guaranteed machine specification.
Build a comparison that another buyer can check
For each offer, record the exact model and bucket configuration, the rated payload, the volume and its measurement basis. Beside them, record the project's loose bulk density evidence and any fill assumption used in the estimate. Keep units consistent. If any of those inputs are missing, flag the resulting mass estimate as incomplete rather than filling the gap with a convenient generic value.
Then give the supplier the relevant material description and application information when requesting a confirmed configuration. Route drawings and receiving-point requirements belong with that inquiry because a capacity match alone does not establish overall suitability. The result should be a traceable comparison: which numbers came from the machine document, which came from the mine, and which remain assumptions.
Keep the calculation linked to the quotation revision
A comparison can become inconsistent after a quotation changes. If the supplier revises the bucket description while the worksheet retains its previous volume, the calculated material mass no longer describes the offer under review. Record the quotation date and configuration reference beside the inputs, and revisit the arithmetic whenever either changes.
Give each estimate a short status such as assumed, project measured or supplier confirmed. These labels explain the strength of the inputs without suggesting that a calculated load is an approved operating load. A reviewer should be able to follow one offer from its stated bucket volume to its density assumption and mass result, then see the separate rated-payload check. This simple document discipline is especially useful when several people exchange revised spreadsheets.
Frequently asked questions
Does a 3 m³ underground loader bucket always carry 7 t?
No. Material mass depends on the loose volume present and its bulk density. The ZDL307 reference lists 3 m³ stacking capacity and 7 t rated load separately. Neither figure makes every bucketful equal to the other.
Can a bigger bucket increase rated payload?
A bucket's volume alone does not establish a higher machine payload rating. Any proposed bucket configuration needs its own confirmation against the applicable machine documentation. Remaining physical space is not permission to exceed rated load.
Which density belongs in an initial loading comparison?
Use project evidence for loose bulk density that represents the material condition being assessed, with units and moisture basis identified. Clearly label preliminary assumptions. Solid-rock density should not be substituted without accounting for the different volume basis.