Understanding Hydro Transmission Systems
Hydro transmission, also known as hydraulic transmission, is a technology that uses pressurized fluid to transmit power from one component to another. This hydro mechanical system is widely used in industrial machinery due to its ability to provide high force and torque with precise control.
In manufacturing environments, hydro transmission systems offer numerous advantages including smooth acceleration, consistent power delivery, and the ability to easily vary speed and direction. The hydro mechanical components work together to convert mechanical energy into hydraulic energy and back, enabling efficient power transfer in complex machinery.
Application in Drilling and Boring Machines
This article focuses on a specific application: a horizontal single-sided drilling and boring combination machine tool. The hydro mechanical transmission system in such machines must handle precise movements and varying loads while maintaining efficiency and accuracy.
The working cycle of this machine follows a specific sequence: "rapid advance — working feed — rapid retract — stop at original position," requiring the hydro transmission system to precisely control speed and force throughout each phase.
Machine Technical Specifications
Operational Parameters
- Maximum axial force: 30 kN
- Self-weight of moving parts: 19.6 kN
- Rapid advance/retract speed: 6 m/min
- Working feed speed range: 0.02~0.12 m/min
Motion Parameters
- Maximum stroke: 400 mm
- Working stroke: 200 mm
- Start/changeover time: 0.2 s
- Flat guide friction coefficient: 0.1
Hydro mechanical transmission system controlling the machine's working cycle
I. Load Analysis and Speed Analysis
1. Load Analysis
The hydro mechanical system must account for various load types during operation. The working load F is 30 kN, while the gravitational load Fg is 0. The inertia load Fi, calculated based on the start-up commutation time and the self-weight of moving parts, is 1000 N, with a frictional resistance Ff of 1960 N.
Taking the hydraulic cylinder mechanical efficiency ηm = 0.9, the load values of the hydraulic cylinder during each working stage are calculated. This analysis is crucial for designing an effective hydro transmission system that can handle all operational demands.
| Working Cycle Stage | Calculation Formula | Load (N) |
|---|---|---|
| Start-up acceleration | F = (Fw + Ff + Fi) / ηm | 32890 |
| Rapid advance | F = Ff / ηm | 2178 |
| Working feed | F = (Fw + Ff) / ηm | 35511 |
| Rapid retract | F = Ff / ηm | 2178 |
2. Speed Analysis
The hydro mechanical system must accommodate significant speed variations. Rapid advance and retract speeds are 6 m/min, while working feed speeds range from 20 to 120 mm/min. This represents a substantial speed range that the hydro transmission system must handle efficiently.
Speed profile throughout the machine's working cycle, showing the wide range handled by the hydro mechanical system
II. Determining Key Parameters of Hydraulic Cylinders
1. Initial Selection of Hydraulic Cylinder Working Pressure
Based on the maximum load values, the working pressure of the hydraulic cylinder is selected as 4 MPa. This pressure rating is optimal for the hydro mechanical system, balancing efficiency, component size, and operational requirements.
The selection of appropriate working pressure is critical in hydro transmission design, as it directly impacts component sizing, energy efficiency, and system performance.
2. Calculating Hydraulic Cylinder Structural Parameters
To achieve equal rapid advance and retract speeds, a single-rod piston cylinder with differential connection is used in this hydro mechanical system. Let the two effective areas of the hydraulic cylinder be A1 and A2, with A1 = 2A2, meaning the rod diameter d = 0.707D (where D is the cylinder bore).
To prevent forward impact when drilling through workpieces, a back pressure of 0.6 MPa is applied to the hydraulic cylinder's return chamber, while a back pressure of 0.5 MPa is used during rapid retraction.
Force Balance Equation for Working Feed Condition:
p1A1 = p2A2 + F
From this, we derive:
A1 = F / (p1 - 0.5p2)
A1 = 35511 / (4×10⁶ - 0.5×0.6×10⁶) m² = 9598×10⁻⁶ m² = 96 cm²
The cylinder bore diameter D is calculated as:
D = √(4A1/π) = √(4×96/π) cm = 11.06 cm
Hydro Mechanical Cylinder Design Considerations
The dimensions calculated ensure that the hydro transmission system can deliver the required forces while maintaining the necessary speed characteristics. The differential connection allows for rapid movement without requiring excessively large pump flow rates.
This careful calculation of hydraulic cylinder parameters ensures that the hydro mechanical system operates efficiently throughout all phases of the working cycle, from rapid advance to precise working feed.
III. Formulating the Hydraulic System Diagram
The design of the hydro mechanical system diagram involves selecting appropriate basic circuits that work together to achieve the desired machine performance. Each circuit in the hydro transmission system serves a specific function while integrating seamlessly with other components.
1. Selecting Basic Circuits
Speed Control Circuit
Since the hydro mechanical system power is relatively small and only handles positive loads, an inlet throttling speed control circuit is used. For better low-speed stability and speed-load characteristics, a speed control valve is employed, with a back pressure set in the hydraulic cylinder circuit. This ensures precise control in the hydro transmission system during the working feed phase.
Pump Supply Circuit
With a ratio of maximum to minimum flow rates of 16:1 and the pump operating at high pressure with small flow for most of the working cycle, a dual pump (or pressure-compensated variable displacement pump) is used in this hydro mechanical system to save energy and improve efficiency.
Speed Changeover and Rapid Circuit
Due to the large difference between rapid advance and working feed speeds, a sequence valve-controlled changeover circuit is selected for smooth transitions in the hydro transmission system. Rapid movement is achieved through a differential circuit, optimizing the hydro mechanical performance during non-cutting phases.
Directional Control Circuit
For smooth direction changes, an electro-hydraulic directional valve is used in this hydro mechanical system. A 3-position, 5-port valve is employed to facilitate mid-position stopping of the hydraulic cylinder and differential connection, providing versatile control options for the hydro transmission system.
2. System Schematic
The complete hydro mechanical system diagram integrates all selected circuits into a cohesive design that meets the machine's operational requirements. Each component in the hydro transmission system is strategically placed to optimize performance and reliability.
Key Components:
- Large flow pump
- Small flow pump
- Electro-hydraulic directional valve
- Speed control valve
- Directional valve
- Hydraulically controlled sequence valve
- Pressure relay
- Filter
IV. Selection of Hydraulic Components
1. Determining Hydraulic Pump and Drive Motor Power
Hydraulic Pump Working Pressure
The maximum working pressure of the hydraulic cylinder in this hydro mechanical system is 4.02 MPa. With a pressure loss of 1 MPa in the inlet line, the maximum working pressure for the small flow pump is 5.02 MPa. The pump's rated pressure should be:
P = (5.02 + 5.02 × 25%) MPa = 6.27 MPa
The large flow pump operates at higher pressure during rapid retraction of the hydraulic cylinder. With a pressure loss of 0.4 MPa in the inlet line during rapid retraction, the maximum working pressure for the large flow pump is:
(1.79 + 0.4) MPa = 2.19 MPa
The pressure setting of the unloading valve should be higher than this value to ensure proper operation of the hydro transmission system.
Hydraulic Pump Flow Calculation
Taking the system leakage coefficient K = 1.2, the minimum pump supply qp is:
qp = K × q1max = 1.2 × 0.5 × 10⁻³ m³/s = 0.6 × 10⁻³ m³/s = 36 L/min
Since the maximum flow required during working feed is 1.9 × 10⁻⁴ m³/s and the minimum stable flow of the relief valve is 0.05 × 10⁻³ m³/s, the minimum flow for the small flow pump is:
qp1 = K × qf + 0.05 × 10⁻³ = 7.28 × 10⁻⁴ m³/s = 4.4 L/min
The minimum flow for the large flow pump is:
qp2 = qp - qp1 = (36 - 4.4) L/min = 31.6 L/min
Determining Hydraulic Pump Specifications
Based on product specifications, a YB1-40/6.3 type double vane pump is selected for this hydro mechanical system. It has a rated speed of 960 r/min and a volumetric efficiency ηv of 0.9. The rated flows of the large and small pumps are 34.56 L/min and 5.44 L/min respectively, which meet the requirements of the hydro transmission system.
Pump Selection Rationale
The selected pump combination provides the necessary flow rates for all operating conditions of the hydro mechanical system while maintaining efficiency. The dual-pump design allows for energy savings during the working feed phase when only the small pump operates at high pressure, while both pumps contribute during rapid movements, optimizing the overall performance of the hydro transmission system.
2. Other Component Selection
In addition to the primary hydraulic pumps, other components of the hydro mechanical system are carefully selected to ensure compatibility, efficiency, and reliability. This includes valves, filters, hoses, and fittings that form the complete hydro transmission system.
Directional Valves
Sized for the maximum flow rate in their respective circuits, ensuring minimal pressure loss in the hydro mechanical system.
Flow Control Valves
Selected to provide precise flow regulation across the required range for the hydro transmission system's working feed speeds.
Pressure Control Valves
Set to maintain system pressures within safe operating limits while providing necessary back pressures for the hydro mechanical system.
Hydraulic Hoses & Piping
Sized to handle maximum flow rates with minimal pressure drop, ensuring efficient operation of the hydro transmission system.
Filters
Selected to maintain oil cleanliness, critical for the longevity and reliability of all hydro mechanical components.
Seals & Fittings
Material-compatible with hydraulic fluids, ensuring leak-free operation of the hydro transmission system under all operating conditions.
V. System Oil Temperature Rise Calculation
The hydro mechanical system spends most of its operating time in the working phase, so temperature rise calculations are based on this operating state. Proper thermal management is crucial for maintaining the efficiency and longevity of the hydro transmission system.
1. Power Calculations
The small flow pump operates at a pressure of 5.02 MPa with a flow rate of 5.33 L/min. Its input power is calculated as 557 W.
The large flow pump is unloaded via an externally controlled sequence valve, with its operating pressure equal to the local pressure loss across the valve. For a valve with a rated flow of 63 L/min and a rated pressure loss of 0.3 MPa, with the large flow pump delivering 33.84 L/min:
Δp = 0.3 × 10⁶ × (33.84 + 44.77 × 5.33/95) / 63 Pa = 0.1 × 10⁶ Pa
The input power of the large flow pump is calculated as 70.5 W.
The minimum effective power of the hydraulic cylinder is:
P0 = F × v = (30000 + 1960) × 0.02 / 60 W = 10.7 W
The heat generated per unit time in the system is:
H1 = Pi - P0 = (557 + 70.5 - 10.7) W = 616.8 W
2. Temperature Rise Calculation
When the tank's height, width, and length ratio is within 1:1:1 to 1:2:3, and the oil level height is 80% of the tank height, the heat dissipation area of the tank is approximately:
A = 6.66√V
Where V is the effective tank volume (m³) and A is the heat dissipation area (m²).
Taking the effective tank volume V = 0.25 m³ and the surface heat transfer coefficient K = 15 W/(m²·℃), the temperature rise is calculated as:
Δt = H1 / (K × A) = 616.8 / (15 × 6.66√0.25) ℃ = 15.6 ℃
Temperature Rise Assessment
The calculated temperature rise of 15.6℃ is within the allowable range for this hydro mechanical system. This ensures that the hydro transmission system will operate within safe temperature limits, preventing oil degradation and maintaining component performance over extended operating periods.
Conclusion
The design of this hydro mechanical transmission system for a horizontal single-sided drilling and boring machine demonstrates the careful engineering required to balance performance, efficiency, and reliability. The hydro transmission system successfully handles the varying loads and speed requirements of the machine's working cycle.
By selecting appropriate components, calculating precise parameters, and ensuring proper thermal management, the designed hydro mechanical system provides the necessary power, control, and efficiency for the machine's operation. The use of a dual-pump configuration optimizes energy usage, while the selected control valves ensure smooth transitions between operating phases.
This hydro transmission system represents a well-engineered solution that meets all the specified requirements, providing a reliable and efficient power transmission method for the drilling and boring machine.