Ground pressure and flotation: contact-area calculations and ground support
Ground pressure and flotation figures describe a stated weight distributed over a stated contact area; they do not establish that a working surface can support a machine. In a simplified example, an assumed 20,000 lb carried on two rectangular track contacts, each 12 inches wide and 108 inches long, gives 7.72 psi average pressure. Increasing the assumed width to 18 inches gives 5.14 psi with the other inputs held constant. Those are illustrations, not specifications or a ground assessment. Deere’s actual 245 P-Tier brochure pairs 700 mm shoes with 45 kPa and 800 mm shoes with 40 kPa, but also lists different operating weights and overall widths for those configurations. Wider shoes can therefore change more than one shortlist entry. Check the supporting ground, loading arrangement and machine instructions separately, including buried structures, edges and weak layers. Low nominal pressure cannot certify a platform, approve tire deflation or guarantee that turning will leave turf undamaged.
Ground pressure and flotation from a stated weight and contact area
| Width per track | Total assumed area | Calculated average |
|---|---|---|
| 12 in | 2,592 in² | 7.72 psi |
| 18 in | 3,888 in² | 5.14 psi |
How is nominal ground pressure calculated for the track example?
Divide the assumed supported weight by the assumed total contact area in compatible units. Two contacts at 12 inches by 108 inches give 2,592 in²; 20,000 lbf divided by that area is about 7.72 psi. This rectangular average omits local roller loading, weight transfer and motion, so it should be labeled as an illustration rather than presented as a measured pressure distribution for a real machine.
What changes in the assumed wider-track calculation?
Holding the assumed 20,000 lb and 108-inch contact length constant while changing each width to 18 inches gives 3,888 in² total area. The calculated average is about 5.14 psi. This isolates one geometric change in a simple model; it does not predict the complete effect of fitting a wider undercarriage, which can also change mass, dimensions, contact behavior and permitted equipment combinations.
Compare the complete Deere shoe configurations
Which two published Deere shoe configurations are being compared?
The 245 P-Tier brochure lists 700 mm triple semi-grouser shoes at 45 kPa, or 6.53 psi, and 800 mm shoes at 40 kPa, or 5.80 psi. Retain the model, shoe type and source configuration with these figures. They are nominal manufacturer entries, not a measured bearing capacity for the site or an instruction that either arrangement can cross a particular slab, trench cover or weak ground.
How do the corresponding machine weights and widths differ?
Deere lists 25,500 kg operating weight and 3.09 m width for the 700 mm shoe configuration, compared with 25,800 kg and 3.19 m for the 800 mm arrangement. The differences are 300 kg and 0.10 m. Do not change only the pressure column in a transport or access comparison; the selected configuration also has to fit the actual weight and width constraints.
What equipment basis accompanies those operating weights?
The brochure’s operating-weight basis includes a full fuel tank, a 79 kg operator, the listed 1.09 m³ heavy-duty bucket, 2.91 m arm and 7,280 kg counterweight. Preserve that basis when comparing another configuration. A different attachment or counterweight can change the load and its distribution, so the shoe-width entry alone cannot identify the nominal ground pressure of every machine carrying the same model badge.
Keep nominal pressure separate from supporting-ground capacity
Why is average pressure different from local pressure?
The machine does not transmit every part of its load as a uniform rectangle under all conditions. Rollers, track structure, attachments and the working position influence the load path, while travel and turning change contact and loading. A nominal average can help compare documented configurations without supplying the peak local pressure or resolving settlement and ground failure at the actual work location.
Does low ground pressure establish adequate soil support?
No single catalog average establishes the strength and condition of the supporting ground. For construction cranes within its scope, OSHA 1926.1402 addresses firm, drained and graded ground and suitable support needed to meet the manufacturer’s requirements. That crane provision is not a soil-rating certificate for every excavator or tractor; assess the actual equipment, ground and applicable requirements for the planned operation.
Which concealed ground conditions need attention?
A sound-looking surface can overlie weak layers, voids, utilities or a buried structure whose capacity differs from the surface appearance. Edges and excavations can also change support conditions. Keep those features in the site assessment rather than assuming that an average psi below a remembered soil figure establishes safe access; an equipment specification does not reveal the site’s subsurface conditions or engineering capacity.
Can a mat’s whole outline be counted as effective area?
A mat must have suitable strength and support for the actual loading arrangement. Its outer dimensions alone do not establish how much area transfers load effectively to the ground or whether local bending and bearing limits are satisfied. Do not divide machine weight by a large arbitrary rectangle and call the result an approved working platform without the required assessment of the mat and supporting surface.
Check tires, turning and the actual working arrangement
Can tire inflation be selected from weight divided by area?
Use the actual tire’s load, speed and application instructions, with its markings interpreted correctly; Michelin explains tire identification and load/speed markings. A simple load-divided-by-pressure relationship is not a complete tire-footprint model. Releasing air until a machine appears to float can violate operating requirements and does not establish adequate tire support, rim compatibility or safe road travel.
Will a low nominal pressure prevent turf damage while turning?
Ground pressure is only one part of the interaction between equipment and a surface. Turning can apply shear and disturb turf even when the nominal vertical average is low, and the actual conditions and operating technique matter. Do not treat a low-pressure label as a guarantee of no surface damage or as permission to operate across ground whose support and access have not been assessed.
Should attachment loads be ignored in a flotation comparison?
Keep the fitted attachment, carried material and working position with the machine configuration. Those loads can alter the supported total and distribution from the catalog basis. A brochure value for one bucket and counterweight does not automatically describe a heavier tool or a loaded bucket; establish the relevant operating configuration rather than subtracting or adding a guessed pressure allowance.
What belongs in the final ground-pressure comparison?
Record model, configuration, operating-weight basis, shoe or tire arrangement, nominal pressure and overall dimensions, then keep the ground assessment as a separate decision. The comparison can identify a configuration worth evaluating without certifying the site. Check access width, transport weight and actual support conditions together; a lower average pressure cannot resolve an exceeded limit or unsupported working arrangement.
Ground-contact and machine specification index · Use the ground-pressure calculator with explicit inputs. Check machine-weight definitions and tire markings before transferring a catalog figure to another configuration.