Chain Pump (Dragon Backbone Pump)
Part of Irrigation
The chain pump is a mechanical water-lifting device that uses an endless loop of paddles or discs pulled through an inclined trough to raise water from rivers, wells, or canals to field level — one of the most effective low-tech irrigation tools ever invented.
The chain pump, known in China as the dragon backbone pump (longgu che), has been in continuous use for over 2,000 years. It was the workhorse of Chinese rice agriculture, capable of lifting water up to 5 meters using nothing more than wood, rope, and human power. Unlike bucket-based systems, the chain pump delivers a continuous flow, making it far more efficient for irrigating large areas.
Operating Principle
An endless chain or rope loop carries a series of evenly spaced paddles (or discs) through a tightly fitted trough. As the chain moves, each paddle pushes a slug of water up the inclined trough. Water enters at the bottom, is pushed upward paddle by paddle, and spills out at the top into a collection channel or directly onto the field.
The key insight is that you are not lifting the full column of water at once. Each paddle only lifts the water between itself and the next paddle — a small amount. But because the chain moves continuously, the cumulative flow rate is substantial.
Components and Materials
| Component | Material Options | Key Requirement |
|---|---|---|
| Trough | Split log, planks, bamboo half-round | Watertight, smooth interior |
| Chain/rope | Twisted rope, linked chain, bamboo strips | Strong, flexible, rot-resistant |
| Paddles/discs | Wood boards, leather on frame, bamboo | Fit trough closely (1-3 mm gap) |
| Axle (top) | Hardwood, iron if available | Smooth rotation, handles crank force |
| Axle (bottom) | Hardwood, greased | Submerged, must resist rot |
| Frame | Timber, bamboo | Rigid, holds trough at correct angle |
| Bearings | Hardwood socket, greased leather | Low friction, replaceable |
| Crank handle | Hardwood dowel | Comfortable grip, sturdy |
Trough Construction
The trough is the most critical component. It must be straight, smooth inside, and fit the paddles as closely as possible. Any gap between paddle and trough wall lets water slip past, reducing efficiency.
Dimensions
| Lift Height | Trough Width (interior) | Trough Depth (interior) | Trough Length |
|---|---|---|---|
| 1-2 m | 10-15 cm | 10-12 cm | 1.5-3 m |
| 2-3 m | 12-18 cm | 12-15 cm | 3-5 m |
| 3-5 m | 15-20 cm | 15-18 cm | 5-8 m |
Building Methods
Split-log method: Find a straight log 20-30 cm diameter. Split it lengthwise and hollow out each half to create a U-shaped channel. Rejoin the halves (keeping the best one) or use a single half as an open-top trough with side boards added.
Plank method: Mill or split boards to 15-20 mm thickness. Assemble a rectangular trough with a flat bottom and two side walls. Seal joints with pine pitch, beeswax, or clay mixed with animal fat. Reinforce with cross-battens every 50 cm.
Bamboo method: For shorter lifts, large-diameter bamboo (8-12 cm) can be split lengthwise and the internal membranes knocked out. Multiple lengths are joined end-to-end.
Trough Straightness
The trough must be dead straight. Any bow, twist, or kink will cause the paddles to jam or create excessive gaps. Sight along the trough before installation and shim or plane as needed. A deviation of more than 3 mm over 2 meters is too much.
Chain and Paddle Assembly
Chain/Rope
The chain must form a continuous loop long enough to run the full length of the trough (inside) plus return (outside), with some slack for the axle wraps.
Total chain length = (trough length × 2) + (circumference of both axle wraps) + 30 cm slack
For a 5-meter trough: approximately 11-12 meters of chain/rope.
Twisted natural fiber rope (hemp, jute, palm) works well but stretches when wet. Account for this by building in adjustment capability. Linked wooden or bamboo chain segments are more dimensionally stable.
Paddle Spacing
Paddles should be spaced 30-50 cm apart along the chain. Closer spacing moves more water per revolution but increases drag and the force needed to turn the crank.
| Paddle Spacing | Water per Minute (approx.) | Crank Effort | Best For |
|---|---|---|---|
| 30 cm | Higher flow | Heavy | Short lifts, strong operators |
| 40 cm | Medium flow | Moderate | General purpose |
| 50 cm | Lower flow | Light | Tall lifts, single operator |
Paddle Construction
Each paddle must fit the trough interior with minimal clearance. Cut paddles from hardwood boards, 15-20 mm thick. The paddle should be 1-3 mm smaller than the trough on each side.
Leather Sealing
Nail a strip of leather around the edge of each wooden paddle, extending 3-5 mm beyond the wood. The leather flexes against the trough walls, creating a better seal than wood alone. This single improvement can increase water output by 30-40%.
Attach paddles to the chain at their center point. Use a hole through the paddle with the chain threaded through, or bolt/peg the chain link to the paddle face. The paddle must be perpendicular to the chain direction when under tension.
Axle and Crank Design
Upper Axle (Drive)
The upper axle sits at the top of the trough and drives the chain. It needs:
- Diameter: 8-12 cm for wooden axles
- Length: trough width + 30 cm (for bearing seats and crank attachment)
- Chain engagement: Grooves, pegs, or a sprocket-like arrangement to grip the chain without slipping
The simplest chain engagement is a hexagonal or octagonal axle cross-section. The chain wraps around it, and the flat faces grip the chain links.
Lower Axle (Idler)
The lower axle sits submerged or at water level. It redirects the chain back up through the trough. Use rot-resistant wood (oak, locust, cedar) and grease it heavily.
Crank Mechanism
Single-person crank: A handle extends from one end of the upper axle, 25-35 cm from center. The operator turns it like a wheel.
Two-person crank: A longer handle extends from both ends, or a crossbar through the axle allows two people to push/pull from opposite sides.
Treadle operation: For sustained use, a treadle (foot pedal) system is less tiring than arm cranking. Build a frame with pedals connected to the axle via a simple crank mechanism. The operator holds a bar for balance and walks/pedals.
Gear Ratio
Adding a simple gear increase (large drive gear on the crank, small gear on the axle) lets you turn a comfortable-speed crank while the chain moves faster. Even a 2:1 ratio doubles water output for the same crank speed. Wooden peg gears work adequately.
Bearing Design
Bearings take the most wear and need regular attention.
Simple socket bearing: Drill or carve a hole in a hardwood block slightly larger than the axle. Grease with animal fat. Replace when the hole elongates.
Split bearing: Two hardwood blocks bolted together around the axle. Can be tightened as wear occurs. Easier to install and replace than drilled blocks.
Greased leather bearing: Wrap the axle contact area in greased leather. Quieter and lower friction than wood-on-wood.
| Bearing Type | Lifespan | Friction | Maintenance |
|---|---|---|---|
| Hardwood socket | 3-6 months | Moderate | Grease weekly, replace when worn |
| Split hardwood | 4-8 months | Moderate | Tighten monthly, grease weekly |
| Leather-wrapped | 2-4 months | Low | Replace leather, grease every 2-3 days |
| Iron bushing (if available) | 1-3 years | Low | Grease monthly |
Frame and Installation
Setting the Angle
The trough angle determines lift height and affects effort and flow rate.
| Angle from Horizontal | Practical Lift | Effort | Flow Rate |
|---|---|---|---|
| 15-20° | 1-2 m | Easy | High |
| 25-30° | 2-3 m | Moderate | Medium |
| 35-45° | 3-5 m | Heavy | Lower |
Steeper angles lift water higher but reduce flow rate and increase the force needed. Above 45°, efficiency drops sharply and water leaks back past the paddles more readily.
Frame Construction
Build a rigid A-frame or tripod structure at the top to hold the upper axle at the correct height. The trough rests on the frame and extends down to the water source. Anchor the bottom of the trough so it stays submerged 10-20 cm below water level.
Cross-brace the frame against lateral movement. The cranking action creates side-to-side forces that can wobble an insufficiently braced frame.
Performance Specifications
| Configuration | Lift Height | Flow Rate | Power Required |
|---|---|---|---|
| 1 person, arm crank | 1-2 m | 5-15 L/min | 50-75 watts |
| 1 person, treadle | 1-3 m | 10-20 L/min | 60-100 watts |
| 2 people, dual crank | 2-4 m | 15-30 L/min | 100-150 watts |
| 2 people, treadle | 3-5 m | 20-40 L/min | 120-180 watts |
| Animal-powered | 3-5 m | 30-60 L/min | 200-400 watts |
At 20 liters/minute, a chain pump delivers 1,200 liters per hour — enough to flood a 100 m² basin to 12 cm depth in one hour.
Operator Fatigue
Sustained chain pump operation is physically demanding. A single person can typically maintain output for 30-60 minutes before needing a break. Plan for relay teams of 2-4 people for day-long irrigation. Treadle operation is less tiring than arm cranking for the same output.
Assembly and Testing
- Build the trough and test it for straightness and watertightness on flat ground
- Assemble the chain with paddles attached at correct spacing
- Install axles in bearings, mount upper axle in the frame
- Thread the chain through the trough and around both axles
- Set the angle and secure the trough in position with the lower end submerged
- Test dry — turn the crank and verify the chain moves smoothly without jamming
- Test wet — lower into water and check for flow, leaks, and paddle fit
- Adjust — tighten chain tension, trim paddles for better fit, grease bearings
Chain Tension
The chain should be taut enough that paddles stay upright in the trough but loose enough that the return side (outside the trough) hangs with a slight sag. Too tight causes excessive bearing wear and makes cranking harder. Too loose causes paddles to tip over in the trough.
Maintenance Schedule
| Task | Frequency | Time Required |
|---|---|---|
| Grease bearings | Every 2-3 days during use | 5 minutes |
| Inspect paddles for wear | Weekly | 15 minutes |
| Check chain for stretch/damage | Weekly | 10 minutes |
| Replace worn paddles | Monthly or as needed | 30-60 minutes |
| Replace bearing blocks | Every 3-6 months | 1-2 hours |
| Repair/reseal trough | Annually | Half day |
| Full chain replacement | Every 1-2 years | 2-4 hours |
Common Failure Points
- Paddle wear: Edges wear down, increasing gap and reducing efficiency. Replace when gap exceeds 5 mm.
- Chain stretch: Rope chains stretch with use. Build in an adjustment mechanism (movable lower axle) to take up slack.
- Bearing seizure: Insufficient greasing causes wood-on-wood friction buildup. A seized bearing can split the axle or frame.
- Trough splitting: Wooden troughs crack along grain lines, especially when drying between uses. Store troughs in shade and keep them damp during dry periods.
Modifications and Improvements
Metal chain and discs: If you have access to blacksmithing, iron chain with iron or wooden discs dramatically increases lifespan and reduces maintenance.
Enclosed trough: Adding a top cover to the trough (making it a tube rather than an open channel) improves efficiency at steeper angles by preventing water from sloshing out.
Multiple stages: For lifts greater than 5 meters, use two chain pumps in series. The first lifts water to an intermediate basin, and the second lifts from there to the field. This is more efficient than a single very steep pump.
Chain Pump Construction Essentials
The chain pump lifts water up to 5 meters using a continuous chain of paddles pulled through a fitted trough. Build the trough straight and smooth (±3 mm tolerance), with paddles fitting within 1-3 mm of the walls — add leather edge strips for a 30-40% efficiency boost. Space paddles 30-50 cm apart on a rope or linked chain. Set the trough at 15-45° from horizontal depending on lift height needed. One person cranking delivers 5-15 L/min at 1-2 m lift; two people on treadles can achieve 20-40 L/min at 3-5 m. Grease bearings every 2-3 days, inspect paddles weekly, and plan for relay operators during extended use. The chain pump is one of the most productive water-lifting devices buildable entirely from wood, rope, and leather.