Bigger Isn’t Always Better: Why Hydraulic Flow and Pressure Matter When Choosing an Excavator Attachment
Choosing the right attachment for your excavator isn’t just about matching it to the machine’s weight. While excavator tonnage is an important starting point, it doesn’t tell you everything you need to know about whether an attachment will perform properly.
Hydraulic flow, operating pressure, motor requirements and the job itself all play a part. Get the combination right and you can achieve reliable performance, efficient operation and a setup that suits your application. Get it wrong, and you could end up with an attachment that underperforms, struggles to do the job or places unnecessary strain on the hydraulic system.
At Autoguide Equipment, we understand that choosing the right equipment means looking beyond the machine's size. Here’s what to consider before investing in a hydraulic attachment.
Why excavator tonnage is only part of the picture
Most excavator attachments are designed to work within a particular machine weight range. This helps establish whether the excavator can safely handle the attachment, including its weight, working forces and any additional loads.
However, two excavators of the same tonnage can have different hydraulic capabilities.
One machine might deliver the flow and pressure an attachment needs, while another may fall short. This is particularly important when choosing hydraulic equipment such as auger drives, torqueheads and other powered attachments.
An attachment that physically fits your excavator isn't necessarily hydraulically compatible with it.
Before choosing equipment, you need to understand three key factors: hydraulic flow, hydraulic pressure and the attachment's motor requirements.
1. Hydraulic flow: how much oil does the attachment need?
Hydraulic flow is the volume of oil delivered to an attachment over a given period. It's usually measured in litres per minute (L/min).
In simple terms, flow influences how quickly a hydraulic motor can operate. The amount of flow an attachment requires depends on its design, motor displacement and intended application.
If the excavator cannot supply enough usable flow, the attachment may operate more slowly than expected or fail to achieve the performance required for the job.
On the other hand, supplying more flow than the attachment is designed to accept can cause excessive operating speed, overheating or damage if the system isn't configured correctly.
The important point: more hydraulic flow isn't automatically better. The attachment must be matched to the flow it is designed to handle.
When checking flow, establish the excavator's available auxiliary hydraulic flow and compare it with the attachment manufacturer's specified operating range. Also check whether the quoted flow is available in the circuit you'll actually be using.
2. Hydraulic pressure: how much force can the system deliver?
Hydraulic pressure is measured in bar and describes the force available within the hydraulic system.
While flow affects motor speed, pressure helps determine the torque or force that a hydraulic attachment can produce, subject to the motor's design and mechanical efficiency.
For applications such as driving screw piles or drilling with an auger, this distinction matters. An attachment may receive sufficient oil flow to rotate at the required speed, but if the system cannot provide the necessary pressure under load, it may struggle to deliver the torque needed for the application.
Equally, an attachment designed for a particular pressure range shouldn't simply be connected to a machine operating at a higher pressure without checking compatibility and the relevant pressure-control settings.
Always check the attachment's maximum permitted pressure alongside its recommended operating pressure. The excavator's maximum hydraulic pressure isn't necessarily the pressure that will be available or appropriate for every attachment.
3. Excavator hydraulic flow and pressure working together
Flow and pressure need to be considered together, rather than as separate figures on a specification sheet.
For hydraulic motor-driven attachments, the combination influences operating speed, available torque and overall performance. The motor's displacement, efficiency and design also affect the result.
This is why two attachments designed for excavators of a similar size can have different hydraulic requirements.
A smaller attachment might need relatively high flow to achieve its intended operating speed, while a different attachment may be designed to operate at lower flow but require higher pressure to deliver the necessary torque.
The correct setup depends on what the attachment is designed to do, not simply how large the excavator is.
4. A worked example: two excavators, the same weight
Imagine you're choosing an auger drive for a 5-tonne excavator.
Your excavator manufacturer lists an auxiliary hydraulic flow of 60 L/min, with a maximum auxiliary pressure of 200 bar. You're considering an attachment with a specified operating range of 40–70 L/min and a maximum permitted pressure of 210 bar.
At first glance, the figures might suggest a suitable match. The excavator's stated flow falls within the attachment's range, and its maximum pressure is below the attachment's stated maximum.
But there's more to check before confirming compatibility.
Available flow: Can the excavator deliver the required flow through the specific auxiliary circuit, under the intended operating conditions?
Working pressure: Is the available pressure sufficient to achieve the required performance under load, rather than simply being below the attachment's maximum limit?
Hydraulic return: Does the attachment require a particular return-line arrangement, such as a low-pressure return, and can the excavator provide it?
Application: Is the attachment designed to deliver the torque and rotational speed needed for the ground conditions and hole diameter involved?
Mounting and weight: Is the attachment suitable for the excavator's lifting capacity, mounting interface and stability requirements?
This example illustrates why a specification sheet is the start of a compatibility check, not the end of one. The figures are illustrative rather than a recommendation for a specific machine or attachment. Final selection should be based on the manufacturers' confirmed specifications and the intended application.
5. Don't overlook the attachment's intended application
Even when the hydraulic figures look right, it's important to consider the work you'll be carrying out.
For example, choosing an auger drive involves more than matching the attachment to the excavator. You'll also need to consider the required hole diameter, drilling depth, ground conditions and the torque needed for the job.
Similarly, when selecting a torquehead for screw pile installation, the required torque, rotational speed and installation method all influence the most suitable setup.
An attachment that performs well in one application may not be the right choice for another, even if both can be operated by the same excavator.
Think about the job first, then work backwards to establish the equipment and hydraulic requirements.
Choosing the right attachment with Autoguide Equipment
At Autoguide Equipment, we manufacture and supply hydraulic equipment for demanding applications, including auger drives and torqueheads.
We know that selecting the right attachment is about more than finding one that fits your excavator. It means understanding the machine, the hydraulic system and the work you need to carry out.
If you're researching an attachment for your excavator, having your machine specifications and application details to hand will help you narrow down the options and identify a suitable setup.
Need help choosing the right attachment? Contact Autoguide Equipment to discuss your application and hydraulic requirements with our team.
Frequently asked questions
What hydraulic flow do I need for an excavator attachment?
The required flow depends on the attachment's design, motor specifications and intended application. Check the manufacturer's recommended flow range and compare it with the usable flow available from your excavator's auxiliary hydraulic circuit.
What is the difference between hydraulic flow and pressure?
Hydraulic flow is the volume of oil delivered over time, measured in litres per minute. Hydraulic pressure, measured in bar, relates to the force the system can generate. For a hydraulic motor, flow influences rotational speed, while pressure helps determine the torque available.
Can I use an attachment designed for a larger excavator on a smaller machine?
Potentially, but it depends on the attachment's weight, mounting requirements, hydraulic specifications and the excavator's capabilities. Tonnage alone isn't enough to establish compatibility. Always check the manufacturer's requirements before use.
What happens if hydraulic flow is too low?
If flow is below the attachment's required range, it may operate more slowly than intended or fail to deliver the performance needed for the job. The impact depends on the attachment design and the work being carried out.
Can too much hydraulic pressure damage an attachment?
Yes. Operating above the attachment's permitted pressure can damage hydraulic components and create safety risks. Pressure must be set and controlled within the manufacturer's specified limits.
Why does hydraulic compatibility matter when choosing an auger drive?
An auger drive needs an appropriate combination of flow, pressure and motor characteristics to deliver the required rotational speed and torque. The correct choice also depends on the excavator, auger diameter, ground conditions and drilling requirements.
How do I check whether an attachment is compatible with my excavator?
Start with the excavator's make and model, available auxiliary flow, operating pressure and hydraulic circuit configuration. Compare these with the attachment's specifications, then confirm the mounting arrangement, machine capacity and intended application with the supplier.
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