Calculating Torque and Gear Reduction for Pulling Heavy Loads with a Gas Engine

If you have built a chassis with an irrigation pump on it and wish to create a new chassis with a 7 HP gas engine to pull it, you need to understand the principles of torque and gear reduction. This guide will help you calculate the necessary setup to efficiently move your heavy load without overpowering your engine.

Understanding Torque and Speed

The speed at which you want to move the load is crucial. A small engine can pull a significant weight if allowed to move slowly enough. For your specific case, with a load of 250 kg, a 7 HP gas engine should be sufficient. A Go-Kart powered by a Honda GX270 motor, which has an output around 8.5 HP, can move at speeds of 60-70 km/h. However, since you only need to pull, not push, at a slower speed, you can indeed use a 7 HP engine.

Calculating Torque and Gear Ratios

First, determine the desired pulling speed. For an easy walking pace of 5 km/h, convert this to meters per second (m/s): 5 km/h 1.39 m/s. Next, calculate the wheel size: 20 cm 0.2 meters, which means the wheel makes approximately 0.6 meters per revolution (a 8-inch diameter wheel).

Desired Wheel Rotations per Second

At 1.39 m/s, the wheel needs to rotate approximately 2-3 times per second (120-180 rpm at the wheel). Aim for the motor to run at around 2700 rpm. Using the motor's power curve, calculate the required gear reduction:

desired speed / wheel circumference diameter * π  desired wheel rotations per second * 60  desired rpmdesired rpm / engine rpm  gear ratio required

Empirical Gear Reduction

Based on these calculations, you would need a gear reduction of around 22 times (2700/120) or 15 times (2700/180). To achieve this, use a two-step reduction setup: a small sprocket drives a larger sprocket, then the second small sprocket drives the large sprocket. For a standard Go-Kart, 14T and 41T sprockets can provide a reduction but might not be enough for 22 times. Alternative sprockets or a gearbox with a 15-25 times reduction should be considered.

Potential Issues

Most issues in such projects stem from the need for sufficient gear reduction rather than insufficient power. Given a 7 HP engine, you have plenty of power, even for a 250 kg load. The critical part is ensuring the gears and sprockets can handle the required torque and RPM.

Conclusion and Recommendations

With careful calculation and the right gear reduction setup, you can successfully build a chassis powered by a 7 HP gas engine to pull your irrigation pump. Ensure you test the setup with a reasonable load to avoid overworking the engine. Additionally, make sure your chassis is well-designed for the intended use to avoid any mechanical issues.

Keywords

torque, gear reduction, gas engine, irrigation pump, chassis