Does the supporting roller's main shaft axially shift out? Don't just tighten the locking nut; check these four clearances.

The axial movement of the supporting roller is a problem that plagues hot rolling, medium and thick plate production lines, and steel profiles production lines.

 

One end of the roller body becomes shiny, the plate shape deviates by one side, and periodic indentations appear at the edge of the steel strip. After stopping the machine for inspection, it was found that the locking nut was loose by half a turn. The maintenance worker's first reaction was: Tighten it back and add some torque.

 

However, within a few days, it loosened again. Tighten it more, then loosen it again. The locking nut became tighter and tighter, and the bearing seat was almost deformed. The roller still shifted.

 

Many teams' approach is "tighten when it shifts, replace the nut when it keeps shifting." But experienced engineers with over a decade of experience know: The root cause of the axial movement of the supporting roller is usually not on the locking nut, but in the other four clearances.

 

Tightening the nut blindly will not solve the problem and may even damage the thrust bearing and loosen the thread of the bearing seat. The cost is higher.

 

Today, let's explain these four clearances in detail - each can be measured and judged on-site, and by following the inspection, you can find the root cause.

 

1. First, understand what we mean by "shifted out":

The supporting roller is designed to allow a small amount of axial movement. Completely locking it tightly is not good - there is no buffer space for thermal expansion, and it will damage the thrust surface.

 

Normal allowable range: The designed axial movement of the supporting roller is generally between 0.3-0.8mm (varies by model, refer to the equipment manual). Within this range, the roller body can have a slight floating movement to release thermal stress.

 

Defective determination signals:

 

Visible gap at the roller body end:

After stopping the machine, push the end of the supporting roller with your hand and feel a significant displacement, exceeding 1-2mm

Rotating marks on the locking nut:

The alignment mark on the end face of the locking nut and the end face of the bearing seat is misaligned, indicating that it has been loosened during operation

Friction marks on the bearing seat end face:

There is abnormal wear on the contact surface between the bearing seat and the window frame, indicating that the bearing seat has shifted inside the window

Periodic edge defects in the plate shape:

The edge of the steel strip has indentations or wavy shapes at regular intervals, corresponding to the rotation cycle of the roller

Any of the above two signals indicate that the axial movement has exceeded the limit and cannot be solved by just tightening the locking nut.

 

2. Four clearances: Follow the inspection sequence

1. Bearing seat and window gap - the most easily overlooked "major loosening"

The supporting roller bearing seat is installed in the frame window, and is positioned by the side and bottom surfaces of the window. If the window is worn, the bearing seat has some movement space inside the window.

 

During rolling, the axial force is transmitted to the frame through the bearing seat. If there is a gap between the bearing seat and the window, the bearing seat itself will have a slight displacement. This displacement adds up to the "shifted out" amount on the roller body.

 

Measurement method:

 

Use a feeler gauge to check the gap between the four sides of the bearing seat and the window, focusing on the axial positioning surfaces of the transmission side and the non-transmission side

If the single-sided gap exceeds 0.5mm, it must be dealt with (add adjustment pads or repair the window)

Check the contact condition between the bottom surface of the bearing seat and the lower crossbeam of the window frame, use a 0.05mm feeler gauge to check, the insertion depth should not exceed 20mm

Common on-site situation: Many frame windows are not repaired for several years, the wear accumulates to 1-2mm, even when the locking nut is tightened to the maximum torque, it cannot be held - because the force is not transmitted to the frame, all is consumed in the gap between the bearing seat and the window.

 

The diaphragm spring is subjected to long-term pressure, resulting in permanent deformation, a decrease in free height, and a reduction in preload force.

There is a single piece of the diaphragm spring that has cracked, and the overall elasticity has decreased.

When installing, the direction of the diaphragm spring is reversed (there are cases where the cone surface is towards the inside or outside), and the deformation after being compressed is incorrect.

Measurement method:

 

Hydraulic locking: Check the system pressure gauge, lock the state and maintain pressure for 5 minutes. The pressure drop should not exceed 10% of the set value.

Diaphragm spring preload: Use a micrometer to measure the compression amount of the diaphragm spring group, compare it with the factory data, and if the attenuation exceeds 15%, it needs to be replaced.

During disassembly and inspection, check if the diaphragm spring has deformed, cracked, or rusted.

?? Judgment rule: If the pressure cannot be maintained, check the seal and pipeline first; if the diaphragm spring cannot be compressed back, check if there are fragments when disassembling.

 

3. The thrust bearing wears or burns out - the "last line of defense" for axial force fails

There are usually thrust bearings (such as thrust ball bearings or thrust roller bearings) at both ends of the supporting roller, specifically designed to bear axial force. If the thrust bearing wears or burns out, the axial movement loses its restriction.

 

Common causes of thrust bearing failure:

 

Poor lubrication: The thrust bearing is located in a remote position, and the lubricating oil/lubricant cannot reach it, resulting in dry friction → burnout

Excessive abnormal axial force: As mentioned earlier, the large window clearance and insufficient locking force led to the thrust bearing bearing loads that it shouldn't have

Excessive pre-tightening during assembly: Pressing the thrust bearing too tightly, the oil film between the rolling elements and the raceway cannot form, leading to early burnout

Judgment method:

 

After stopping the machine, manually rotate the roller. If there is a "clicking clicking" sense when pushing axially, it indicates that there is wear inside the thrust bearing

During disassembly, check if the thrust bearing raceway has flaking, if the rolling elements are shattered, and if the cage is deformed

Check if the positioning surface of the thrust bearing seat has indentations or deformation

⚠️ Key reminder: If the thrust bearing has burned out, only replacing the bearing without addressing the previous window clearance and locking force issues will cause the new bearing to fail in a few months. It is necessary to first solve the root cause before replacing the bearing.

 

4. Thermal thrust generated by the temperature difference of the roller body - "invisible force"

During the rolling process, there is a temperature difference between the surface and core of the supporting roller, and there may also be a temperature difference at both ends. Uneven thermal expansion will generate axial thermal stress.

 

Under what circumstances is the thermal thrust particularly large:

 

Unstable rolling rhythm: Fast and slow, with large temperature fluctuations of the roller body

Uneven cooling: Clogged or uneven distribution of the cooling water nozzles for the rollers, with one end of the roller body cooled well and the other end cooled poorly

After changing the roller, when it is put into operation: The new roller has a low temperature, and suddenly being put into rolling, the thermal shock is large

The size of the thermal thrust can reach tens of thousands of Newtons or even hundreds of thousands of Newtons. If the previous locking force and the state of the thrust bearing are normal, this force can be held. However, if there is a gap in the window or the locking force has weakened, the thermal thrust will push the roller "out".

 

Relief method:

 

After changing the roller, operate at low speed with a hot roller first, allowing the roller body to heat uniformly, and do not directly operate at high speed

Check the cooling water system to ensure uniform cooling along the entire length of the roller body

Before stopping, gradually reduce speed and load to allow the roller body to cool uniformly, do not stop abruptly

 

Three, processing sequence: Measure first, then tighten, do not blindly increase torque

The sequence of inspection determines efficiency and effect. Follow the following sequence:

 

1. Check the temperature difference between the surface and core of the roller body

2. Check the cooling water system to ensure uniform cooling along the entire length of the roller body

3. Check the window clearance and locking force state of the thrust bearing

4. Disassemble and inspect the thrust bearing for deformation, cracks, and rust

5. Check the positioning surface of the thrust bearing seat for indentations or deformation

 

Steps

 

Inspection items

 

Standards / Methods

 

Judgment

 

1

 

Clearance between bearing housing and window

 

Use a feeler gauge to check the four sides. The single-side value should be ≤ 0.5mm

 

Out of tolerance → Add spacer or repair the window

 

2

 

Locking cylinder/disc spring condition

 

Pressure holding test / Micrometer measurement of compression amount

 

Decay exceeds 15% → Replacement

 

3

 

Thrust bearing condition

 

Manual rotation of the roller to check the gap sensation, disassemble and inspect the raceway

 

If there is peeling/fracturing → Replace the entire set

 

4

 

Cooling uniformity

 

Visual inspection of the nozzle, using a thermometer to measure the temperature difference between the roller body

 

If the temperature difference exceeds 15℃ → adjust the cooling

 

Handling principle:

 

First, solve the gap problem (window, locking), then replace the bearing

The torque of the locking nut should be carried out according to the equipment manual. Do not blindly increase it. Excessive torque → excessive bearing preload → overheating → burn damage

Record the data after each treatment and establish trend tracking. When stringing again next time, compare the historical data to quickly locate the cause

 

Four, summary in one sentence

The supporting roller is axially displaced. Don't rush to tighten the locking nut.

 

First, check if the gap is large, then verify if the locking force is sufficient, then see if the thrust bearing is good, and finally think about whether the thermal thrust is the culprit.

 

If these four steps are done properly, it is more effective than breaking ten locking nuts.