Manure spreader draft

[Tim Harrigan]

The recent comment by Does’ Leap in the How Much is Too Much thread (Horses) caused me to revisit some work in evaluating manure spreader draft. Compared to most implements spreader draft is not very predictable and is quite variable with the size and fill of the spreader, density of the manure, long straw or chopped straw bedding, and the surface over which the spreader is drawn.

Ground-driven spreaders have two control levers: one to activate the apron chain and a second to engage the beaters. The beater lever acts as a clutch to connect and disconnect power from the drive wheels by raising or lowering a chain that connects the beaters with the large drive sprocket on the main axle. The apron control lever engages a drive cam assembly on the axle attached to a ratchet gear on the rear apron shaft. The apron control lever acts as a transmission by allowing the selection of a range of apron chain speeds.

What we did
At Tillers International in Scotts, MI we measured the draft of a New Idea #17 and a John Deere #51 series L spreader. When struck level the NI capacity was 48 bu and the JD was about 38 bu. Manure capacities were always given as ‘heaped’ volumes, typically stuck level plus 40-50%. So the official capacity of the NI was probably 65-70 bu and the JD was likely 50-55 bu. The drive wheels on the NI were 7.5×24 pneumatic tires inflated to 30 psi and it weighed 1,060 lb empty. The JD drive wheels were 7.5×20 at 30 psi and it weighed 1,050 lb empty.

We pulled the spreader with a Pioneer forecart (350 lb) with the spreaders struck level with bedded-pack straw manure that was lightly soiled (16 lb/ft-cu), moderately soiled (25 lb/ft-cu) and heavily soiled (35 lb/ft-cu). For reference, non-bedded manure weights about 60 lb/ft-cu. When struck level with lightly soiled bedding the weight of the forecart, spreader and bedding was 2,064 lb for the JD and 2,256 lb for the NI. The draft from rolling resistance was largely influenced by the ground surface, and the draft of the JD was generally a little greater as a percentage of the total weight, likely because the drive wheels were a little smaller (7.5×20 vs. 7.5×24 NI).

Rolling resistance
As expected, spreader draft (struck level, lightly soiled bedding) increased when going from the gravel road to the softer surfaces of a hay field, corn ground and plowed ground. Spreader draft from rolling resistance alone without spreading manure was:
1) gravel road, NI, 98 lbf, 4% GVW; JD, 106 lbf, 5% GVW,
2) hay ground, NI 161 lbf, 7% GVW; JD 178 lbf, 9% GVW,
3) corn ground, NI, 187 lbf, 8% GVW; JD, 215 lbf, 10% GVW, and
4) plowed ground, NI, 544 lbf, 24% GVW; JD 473 lbf, 23% GVW.
Notice that draft from rolling resistance alone increased about 5X going from the gravel road to the plowed ground.

Mechanical resistance
Mechanical resistance with a spreader is the power absorbed by the spreader when operating the beaters and apron chain under no-load to overcome the friction of moving parts. Mechanical resistance was measured as the average draft of an empty spreader with the apron chain and beaters engaged, minus the draft from rolling resistance. The mechanical resistance of each spreader added only about 20 lbf the implement draft.

Material resistance
The material resistance was the power absorbed in the unloading of the manure: 1) advancing the load with the apron chain, 2) tearing manure and bedding from the load, and 3) distributing it over the field. The material resistance was largely influenced by the density and the feed rate (ft-cu/min) with which it was processed by the beaters. A typical ground speed was about 2¾ mph. Based on the apron chain speed notch selected the feed rate for the NI ranged from 16 to 43 cfm when struck level and 24 to 60 cfm heaped. The JD ranged from 11 to 41 cfm level and 15 to 57 cfm heaped. The manure feed rate (cfm) was calculated based on the travel speed (mph), the rate of apron chain advance (feet per foot of ground travel) and spreader volume (cubic feet). At any given feet rate the lbs of manure spread depended on the density of the manure (lightly soiled, heavily soiled etc.).

Spreader draft.
Spreading draft included resistance from rolling, mechanical and material flow and was measured in the spring on hay ground. A wide range in draft was measured depending upon the feed rate and density of manure in the load, and whether the load was level or heaped. A heaped load increased both the rolling resistance (heavier load) and the material resistance (higher feed rate even though the apron chain speed was unchanged). I will focus on the draft of the NI.

When the NI was struck level and set to apply a ‘light’ application (notch 3, 16 cfm) of lightly soiled bedding, an average draft was 240 lbf. A ‘heavy’ (notch 7, 43 cfm) application of the same material increased draft 66% to 399 lbf. A ‘light’ application of heavily soiled bedding (notch 3, 16 cfm, 35 lb/cu-ft) increased draft 76% (to 423 lbf). A ‘heavy’ application of heavily soiled bedding (notch 7) increased draft to 772 lbf.

Heaping the spreader with heavily soiled manure had a big impact on draft. Compared to a ‘light’ application of the lightly soiled manure (311 lbf), draft increased to 579 lbf with a light application of heavily soiled manure. At the high feed rates (notch 7) draft ranged from 524 lbf for the lightly soiled manure to 770 lbf with the moderately soiled manure. We did not measure the heavily soiled bedding draft at the moderate or high feed rates with a heaped load from fear of plugging the spreader or breaking ground drive wheel traction.

Working considerations
An understanding of how ground conditions, size of the load, type of bedding, manure density and material feed rate influence spreader draft makes it possible to match the loads with the draft power available. Heaping a spreader with heavily soiled manure and selecting a high feed rate can stress both the animals and the spreader. High feed rates can plug the beaters and cause the drive wheels to lose traction, and may cause broken chains, bent apron slats or other mechanical problems.

Manure spreaders can be managed in ways that ease the burden of your team: 1) when loading the spreader, do not pile manure against the beaters, and make sure the beaters are able to turn freely before you engage the apron chain, 2) reduce the size of the load when spreading a heavily soiled manure and bedding mix, 3) select a slow feed rate for the apron chain, 3) use pneumatic rather than steel tires, and 4) spread smaller loads on soft or tilled ground, and spread downhill when possible.

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