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Hydraulic vs Mechanical Expansion Sleeves: Which is Best for Your Needs?

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Traditional hub connection devices typically rely on keys to join the hub and shaft, with keyways machined into both components.

Unfortunately, this practice weakens the shaft’s strength, increases manufacturing costs, and can even make machining more difficult.

To simplify the installation process, some engineers prefer keyless connections, where the hub and shaft are interference-fitted. During installation, the hub mounting hole expands through heating, or the installation shaft shrinks through cooling, thus creating the necessary gap to fit the components together.

Yet, this method presents problems when disassembling the components later on.

With the development of technology, keyless connections between hubs have emerged that rely on tapered interference fit to achieve installation and torque transmission.

Mechanical expandable sleeves are inserted between the shaft and the hub, and tightening screws are used to make the two mating tapered surfaces relatively move against each other, creating an interference fit between the mating surfaces of the two components.

This generates a strong clamping force between the hub and shaft, enabling a keyless connection.

When load is applied, the expandable sleeve, combined with the clamping pressure and frictional force generated between the hub and shaft, transmits torque, axial force, or combined loads.

However, traditional mechanical expandable sleeves have the following issues during use:

  1. Mechanical expandable sleeves consist of at least two components, and some even have up to five, making their manufacturing process very complex.
  2. Mechanical expandable sleeves require symmetric tightening during installation, which involves using a torque wrench to tighten screws in stages according to 1/4T, 1/2T, 3/4T, and 1T of the rated torque. This process is time-consuming and labor-intensive.
  3. As the mechanical expandable sleeves age, the tapered surfaces between them rust more easily, making disassembly challenging.
  4. Due to the relative sliding between mating tapered surfaces, traditional mechanical expandable sleeves’ connection method is prone to wear, especially when frequently assembled and disassembled. Furthermore, their assembly environment requires high cleanliness levels, as even small dust particles can cause surface wear.
  5. The wall thickness of mechanical expandable sleeves is uneven, and after bolts are tightened, the thin end’s edges will experience stress concentration, leading to cracks or even breakage of the shaft.

Fortunately, in recent years, a hydraulic expandable sleeve has emerged as an excellent solution to the limitations of mechanical expandable sleeves.

Like mechanical expandable sleeves, hydraulic expandable sleeves are keyless connecting devices. Still, their structure is entirely different, primarily composed of the expandable sleeve body, regulating screw, piston, sealing ball, sealing screw, and hydraulic oil.

The expandable sleeve body’s structure is complicated, consisting of a flange end and an inner and outer cylinder, which encircle a hydraulic chamber for storing hydraulic media that communicates with the piston chamber inside the flange end.

The specific structure is shown in the figure below.

Hydraulic expansion sleeve

1-Hydraulic medium
2-O-ring
3-Pressurized piston
4-Pressure regulating screw
5-Sealing screw
6-Seal ball
7-Expansion sleeve main body

Working principle of hydraulic expandable sleeves:

Inject hydraulic media into the sealed hydraulic chamber and tighten the regulating screw to compress the volume of the hydraulic media, creating a uniform hydrostatic pressure.

This pressure causes the inner and outer cylinders to deform elastically, causing the inner cylinder to shrink and the outer cylinder to expand.

As the inner and outer cylinders tightly fit with the shaft and hub, the expandable sleeve continues to be compressed until it contacts the hub and shaft to generate positive contact pressure.

The load is then transmitted via the frictional force generated at the contact surface. When the regulating screw is loosened, the hydraulic chamber pressure gradually decreases to zero, and the inner and outer cylinders quickly return to their initial state elastically, separating the expandable sleeve from the shaft and hub.

In this way, rapid disassembly of the shaft and hub hole can be achieved.

The working state is shown in the figure below.

Schematic Diagram of Hydraulic Expandable Sleeve

Characteristics of hydraulic expansion sleeve

In addition to the advantages of traditional mechanical expandable sleeves, hydraulic expandable sleeves also have their own unique characteristics:

Faster installation and disassembly.

By compressing the liquid to generate high hydrostatic pressure, the hydraulic expandable sleeve contacts the hub and shaft to transmit the load through contact pressure.

The hydraulic pressure can be produced with only one screw, which makes its installation and disassembly efficiency at least six times higher than that of tapered roller bearings, as shown in Figure (a).

High positioning accuracy.

Traditional mechanical expandable sleeves generate axial force during pre-tightening, affecting axial positioning accuracy.

In contrast, hydraulic expandable sleeves rely on contact pressure generated by contact surfaces to transmit frictional torque and axial loads, not only improving axial positioning accuracy but also having unmatched advantages in situations with high requirements for axial positioning accuracy, as shown in Figure (b).

Space-saving along the axial direction.

By designing a flange end that can accommodate regulating screws and pistons, hydraulic expandable sleeves break through the constraints of the conventional expandable sleeve structure and allow for radial installation and disassembly, without requiring axial operation space for the expandable sleeve, as shown in Figure (c).

Small radial pulsation.

Traditional mechanical expandable sleeves consist of an inner and outer sleeve with a taper, while the hydraulic expandable sleeve is a completely integrated structure, greatly reducing the hub’s radial pulsation during operation, as shown in Figure (d).

Through the above advantages and working principles, we can recognize that hydraulic expandable sleeves have unique advantages in design, use, and disassembly, making them irreplaceable by traditional expandable sleeves.

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