Aux 2 Unit 2 Q2

Core Principle

• In such a system, a hydraulic pump maintains a consistent pressure within the fluid lines, regardless of whether the crane is actively lifting or not. This contrasts with variable-displacement systems where the pump output adjusts to the load.  

Key Components and Their Functions:

• Hydraulic Pump: Usually a gear pump, it delivers a constant flow of hydraulic fluid. Even when no crane function is in use, this pump keeps the system pressurized.  
• Control Valves: These direct the flow of pressurized fluid to the various cylinders that power the crane’s movements (luffing, slewing, hoisting, etc.).  
• Actuator Cylinders: These convert hydraulic pressure into mechanical force, extending or retracting their piston rods to move the crane’s parts.  
• Relief Valve: A safety device that opens if pressure exceeds a safe limit, diverting fluid back to the tank to prevent damage.  

Operational Steps:

1. Idle State:
   • The pump runs continuously, maintaining system pressure.  
   • Control valves are in neutral, blocking fluid flow to the cylinders.
   • Excess fluid flows over the relief valve back to the tank, dissipating energy as heat.
2. Operator Initiates a Function (e.g., hoisting):
   • They manipulate the corresponding control valve.
   • This opens a path for pressurized fluid to flow to the hoist cylinder.
   • The cylinder extends, lifting the load.
3. Load is held or moved:
   • As long as the valve is held, fluid keeps flowing, maintaining pressure and position/movement.
   • If the load needs to be held stationary, the valve is carefully adjusted to balance the flow in/out of the cylinder.
4. Function is stopped:
   • The operator returns the control valve to neutral.
   • Fluid flow to the cylinder stops.
   • Check valves trap the fluid in the cylinder, holding the load in place.
   • The pump’s output once again goes mainly over the relief valve.

Advantages of Constant Pressure Systems:

• Simplicity: Generally simpler design and control compared to variable displacement systems
• Rapid Response: Can provide quick initial movements as pressure is always available.
• Suitable for Multiple Functions: Can easily power several crane functions simultaneously, as long as the pump’s capacity is sufficient.

Disadvantages:

• Energy Inefficiency: Significant energy is wasted as heat when the crane is idle or lightly loaded.
• Heat Generation: Can lead to higher operating temperatures, necessitating efficient cooling systems.
• Less Precise Control: Holding a load perfectly still or achieving very slow, controlled movements can be trickier compared to variable systems.

Applications:

• Commonly found in smaller to medium-sized cranes where simplicity and cost are priorities.
• Often favored in applications where the crane is frequently in use, reducing the impact of idle energy losses.

Remember, this is a simplified overview. Actual crane hydraulics can be much more complex, involving additional valves, accumulators, and sophisticated control systems.

A unidirectional, fixed displacement pump is a type of hydraulic pump that has two key characteristics:

1. Unidirectional: It only pumps fluid in one direction. If the input shaft rotates in the opposite direction, the pump will not produce any flow.
2. Fixed Displacement: It delivers a constant volume of fluid per revolution of its input shaft. This means that the flow rate is directly proportional to the pump’s rotational speed, and it cannot be adjusted while the pump is running.  

Common types of unidirectional, fixed displacement pumps:

• Gear Pumps: These pumps use two meshing gears to trap fluid between the gear teeth and the pump casing, transferring it from the inlet to the outlet.  
• Vane Pumps: These pumps have vanes that slide in and out of slots in a rotor. As the rotor turns, the vanes trap fluid between them and the pump casing, moving it from the inlet to the outlet.  
• Some Piston Pumps: While many piston pumps offer variable displacement, there are also fixed-displacement versions where the piston stroke length is constant.  

Advantages:

• Simple and Reliable: These pumps tend to have a simple design with fewer moving parts, making them reliable and easy to maintain.
• Cost-Effective: Due to their simplicity, they are generally less expensive than variable displacement pumps.
• Suitable for Constant Flow Applications: They are well-suited for applications where a constant flow rate is required, such as lubrication systems or simple hydraulic circuits.

Disadvantages:

• Limited Control: They lack the ability to adjust the flow rate, which can be a disadvantage in applications where varying flow demands are needed.
• Energy Inefficiency: In applications where the flow demand varies, these pumps can be less energy-efficient, as excess flow may need to be bypassed or throttled, leading to energy losses.

Applications:

• Lubrication Systems: To supply a constant flow of lubricating oil to engine bearings and other moving parts.
• Simple Hydraulic Circuits: In applications where a constant flow rate is sufficient, such as powering simple hydraulic cylinders or motors.
• Transfer Pumps: For transferring fluids at a constant rate from one location to another.

In conclusion, a unidirectional, fixed displacement pump is a simple and reliable pump type that delivers a constant flow of fluid in one direction. It’s a cost-effective solution for applications with consistent flow requirements but may be less efficient in situations where the flow demand varies.

In a crane operated by a constant pressure hydraulic system with unidirectional, fixed displacement pumps running continuously, the accumulator serves several crucial purposes:

1. Energy Storage and Peak Demand:

• Storing Energy: The accumulator acts as a reservoir of pressurized hydraulic fluid, storing potential energy that can be released quickly when needed.  
• Meeting Peak Demands: During operations requiring high flow rates for a short duration, such as rapidly extending or retracting a cylinder, the accumulator supplements the pump’s output, ensuring adequate flow without the need for a larger, more expensive pump.  

2. Maintaining Pressure and Compensating for Leaks:

• Pressure Stabilization: The accumulator helps maintain a relatively constant pressure in the system, even when the pump’s output momentarily falls short of demand or there are minor leaks in the system. This ensures smooth and consistent operation of the crane.  
• Leak Compensation: In the event of minor leaks, the accumulator can release stored fluid to compensate for the lost volume, preventing a sudden drop in system pressure and potential operational disruptions.  

3. Dampening Pressure Pulsations and Shocks:

• Smoother Operation: The accumulator absorbs pressure pulsations caused by the pump’s reciprocating action or sudden changes in flow demand. This helps smooth out pressure spikes, reducing wear and tear on system components and providing a more stable and controlled operation of the crane.  
• Shock Absorption: It also acts as a shock absorber, dampening sudden pressure surges caused by rapid valve closures or unexpected movements of the crane. This protects the system from damage and ensures smoother operation.  

4. Emergency Power Source:

• Backup Power: In the event of a power failure or pump shutdown, the accumulator can provide a limited reserve of pressurized fluid to allow for controlled lowering of the load or other emergency operations. This can prevent accidents and ensure the safety of personnel and equipment.

Specific Advantages in Constant Pressure Systems:

• Energy Efficiency: While constant pressure systems are inherently less energy-efficient than variable displacement systems, the accumulator helps mitigate this by storing energy during low-demand periods and releasing it during peak demands, reducing the need for continuous high-pressure pump operation.
• Simplified Control: In simpler crane designs, the accumulator can eliminate the need for complex control valves or variable displacement pumps, reducing system complexity and cost.

In conclusion, the accumulator is a crucial component in a constant pressure hydraulic system for crane operation. It enhances the system’s efficiency, responsiveness, and safety by storing energy, maintaining pressure, dampening shocks, and providing a backup power source in emergencies.

In a constant pressure hydraulic system powering a crane, where the pumps run continuously and have fixed displacement, pressure regulation is primarily achieved through the use of a relief valve in conjunction with the system’s design and operational characteristics.

Relief Valve:

• Function: The relief valve is a safety and control device that limits the maximum pressure in the hydraulic system.  
• Operation: It is set to a predetermined pressure level. When the system pressure exceeds this set point, the relief valve opens, allowing excess fluid to bypass back to the tank. This prevents over-pressurization and potential damage to components.  
• Constant Pressure: In a constant pressure system, the relief valve plays a crucial role in maintaining the desired pressure level. When the crane is not performing any work (i.e., control valves are in neutral), the pump’s continuous flow is diverted through the relief valve, maintaining the set pressure while dissipating excess energy as heat.

System Design and Operational Characteristics:

• Pump Capacity: The pump is selected to provide sufficient flow to meet the maximum anticipated demand of the crane’s operations. However, during periods of low demand or idle states, excess flow is generated.
• Control Valves: Directional control valves regulate the flow of pressurized fluid to the various crane functions (luffing, slewing, hoisting, etc.). When these valves are in neutral, they block the flow, causing the pump’s output to be diverted through the relief valve.
• Accumulator (Optional): An accumulator can be incorporated into the system to store pressurized fluid and supplement the pump’s output during peak demand periods. This helps reduce pressure fluctuations and ensures adequate flow for rapid movements.

Pressure Regulation Process:

1. Pump Operation: The fixed displacement pump runs continuously, generating a constant flow of fluid at a relatively high pressure.
2. No-Load/Idle Condition: When the crane is not performing any work, all control valves are in neutral. The pump’s output is diverted through the relief valve, which opens to maintain the set pressure level. Excess fluid flows back to the tank, dissipating energy as heat.
3. Crane Operation: When the operator activates a control valve, pressurized fluid flows to the corresponding actuator cylinder, performing the desired function (e.g., lifting a load). The system pressure may drop slightly due to the increased flow demand.
4. Pressure Maintenance: The relief valve senses the pressure drop and adjusts its opening to maintain the set pressure. The pump continues to deliver a constant flow, and any excess flow not required by the crane’s operation is still bypassed through the relief valve.
5. End of Operation: When the operator releases the control valve, the flow to the cylinder stops, and the system pressure rises again. The relief valve further adjusts its opening to maintain the set pressure, and excess flow is diverted back to the tank.

Important Considerations:

• Relief Valve Setting: The relief valve setting is crucial for determining the maximum system pressure and should be adjusted carefully based on the crane’s design and operational requirements.
• System Efficiency: While constant pressure systems offer simplicity and quick response, they can be less energy-efficient than variable displacement systems due to the continuous energy dissipation through the relief valve during idle periods.
• Heat Generation: The continuous operation of the pump and the relief valve can generate significant heat, requiring adequate cooling systems to maintain safe operating temperatures.

In conclusion, in a constant pressure hydraulic system for crane operation, pressure regulation is primarily achieved through the relief valve, which acts as a pressure-limiting device and maintains a consistent pressure level regardless of the flow demand. The system design and operational characteristics also contribute to pressure regulation by ensuring adequate pump capacity and controlling fluid flow through directional control valves.

In a crane with a constant pressure hydraulic system using unidirectional, fixed displacement pumps, the speed and direction of the hoist motor are controlled primarily through the use of flow control valves and directional control valves.

1. Speed Control:

• Flow Control Valve: The primary method for varying the hoist motor’s speed is by using a flow control valve. This valve restricts or regulates the flow of pressurized fluid to the hoist cylinder, thereby controlling the speed at which the piston extends or retracts.
• Variable Orifice: The flow control valve typically has a variable orifice that can be adjusted to increase or decrease the flow area. A smaller orifice restricts the flow, leading to slower motor speed, while a larger orifice allows for faster movement.
• Meter-In or Meter-Out Control: Flow control valves can be configured for either meter-in or meter-out control. Meter-in control regulates the flow into the cylinder, while meter-out control regulates the flow out of the cylinder. The choice depends on the specific application and desired control characteristics.

2. Direction Control:

• Directional Control Valve: A directional control valve is used to change the direction of the hoist motor’s rotation, thus controlling whether the load is being lifted or lowered.
• Spool Valve: The most common type of directional control valve used in cranes is a spool valve. It has a sliding spool that can be moved to different positions to connect or block fluid passages, thereby directing the flow to either side of the hoist cylinder.

Operational Procedure:

• Hoisting (Lifting): The operator moves the directional control valve to a position that allows pressurized fluid to flow to the rod-end side of the hoist cylinder, causing the piston to extend and lift the load. The flow control valve regulates the flow rate, controlling the hoisting speed.
• Lowering: The operator moves the directional control valve to a position that allows pressurized fluid to flow to the piston side of the hoist cylinder. This causes the piston to retract, lowering the load. Again, the flow control valve regulates the lowering speed.
• Holding: To hold the load stationary, the operator carefully adjusts the directional control valve to a neutral position or a position that balances the flow in and out of the cylinder, maintaining the desired load height.

Additional Considerations:

• Load Sensing: Some advanced systems may incorporate load-sensing valves that automatically adjust the pump’s output flow based on the load on the crane, optimizing efficiency and reducing energy wastage.
• Regenerative Circuits: Regenerative circuits can be used to recover energy during lowering operations, further improving efficiency.
• Counterbalance Valves: Counterbalance valves are often included in the circuit to prevent the load from dropping uncontrollably in case of a hose or pipe failure.

In summary, the speed and direction of the hoist motor in a constant pressure hydraulic crane system are controlled through a combination of flow control valves and directional control valves. The flow control valve regulates the flow rate to the hoist cylinder, thereby controlling the motor’s speed, while the directional control valve determines the direction of fluid flow, enabling lifting or lowering of the load.

In a constant pressure hydraulic system for a crane, where the pump’s output is fixed, the torque available from the hoist motor is primarily varied by controlling the pressure of the hydraulic fluid reaching the motor. This is because the torque output of a hydraulic motor is directly proportional to the pressure difference across its inlet and outlet ports.

Here’s how this is typically achieved:

1. Pressure Reducing Valves:

• These valves are installed in the hydraulic line leading to the hoist motor.
• They can be manually or automatically adjusted to reduce the pressure of the fluid reaching the motor.
• Lower pressure results in lower torque output from the motor.

2. Variable Displacement Motor (If Present):

• Some cranes may employ a variable displacement hydraulic motor for the hoist function.
• In this case, the motor itself can change its displacement (the volume of fluid it requires per revolution), directly affecting its torque output.
• Larger displacement = higher torque at the same pressure.

3. Indirect Control through Flow:

• While not a direct control of torque, the flow control valve used to regulate hoist speed also indirectly affects torque.
• At a given pressure, a higher flow rate allows the motor to spin faster, potentially generating more torque (depending on the motor’s characteristics).
• However, there’s a trade-off: if the load is too heavy, increasing flow might not lead to higher speed, but simply cause the relief valve to open, limiting pressure and thus torque.

Operational Scenario:

• Lifting a heavy load:
  • The operator might initially open the directional control valve fully, allowing maximum pressure to the motor for maximum torque.
  • Once the load is moving, they might adjust the pressure reducing valve or (if present) the motor’s displacement to achieve the desired lifting speed while maintaining sufficient torque.
• Precise placement of a light load:
  • The operator would likely use a lower pressure setting or smaller motor displacement to avoid jerky movements and achieve fine control.
  • The flow control valve would also be adjusted to limit speed.

Important Considerations:

• System Pressure Limits: It’s crucial to never exceed the maximum pressure rating of any component in the system.
• Load and Safety: The operator must always consider the load’s weight and the crane’s capacity to ensure safe operation.
• Efficiency: While constant pressure systems are simpler, they can be less energy-efficient, especially when operating at reduced torque/speed.

In Summary:

While the pump in a constant pressure system delivers a fixed flow, the torque of the hoist motor can still be varied by controlling the pressure reaching it, either through pressure reducing valves or, in some cases, by using a variable displacement motor. This allows for a degree of control over the crane’s lifting capabilities and operational finesse.