Plastic Pelletizing, Pipe, Profile & Sheet Extrusion – Which Gearbox Handles High Axial Thrust?

Why Extruder Gearboxes Fail – and How to Prevent It

You have a single‑screw extruder line – for pelletizing, pipe, profile, or sheet. The screw turns. The material melts and pushes forward. That forward push creates an axial force pushing back against the screw. That force travels through the coupling, into the gearbox output shaft, and then directly into the gearbox bearings.

Standard helical gearboxes (R, K, F series) are not designed for this. Their output bearings are sized for radial loads from belts or chains, not for continuous axial thrust.

The result? Bearing noise within months. Shaft seal leaks. Then a complete gearbox failure – and days of lost production.

At HelmeDrive, we have worked with extruder lines in high-growth manufacturing markets. This guide explains why the ZLYJ series – a helical gearbox with a dedicated thrust bearing package – is the correct choice for single‑screw extruders in pelletizing, pipe, profile, and sheet applications.

At a Glance: What Makes ZLYJ Different from Standard Gearboxes?

The key is the integrated thrust bearing assembly. ZLYJ gearboxes are built to absorb the full axial force generated by the extruder screw, directly inside the gearbox output housing. Standard helical units cannot do this.

1. Plastic Pelletizing Line (Compounding / Masterbatch / Recycling)

The application: A single‑screw extruder melts and mixes polymer with additives, then pushes the melt through a die. A rotating cutter (dry or underwater) chops the strands into pellets.

Why axial thrust is high: The screw must push molten polymer through a screen pack and a die with multiple holes. Back pressure can reach 200‑400 bar. That pressure translates directly into axial thrust on the screw – and on the gearbox output shaft.

Consequences of wrong gearbox choice: A standard helical gearbox will lose bearing preload within months. Seal leakage follows. Production stops.

ZLYJ advantage: The thrust bearing package is sized for continuous high axial load. Many pelletizing lines run 24/7 – ZLYJ can be ordered with cooling coils for hot ambient conditions.

Typical power range: 55 kW – 250 kW (depending on extruder size and output rate)

2. Plastic Pipe Extrusion (PE / HDPE / PVC / PPR)

The application: Single‑screw extruders for water pipes, gas pipes, conduit, and drainage pipes. Screw diameters from 45 mm to 150 mm.

Why axial thrust is high and what happens when you choose wrong: Pipe extrusion requires high melt pressure (250‑350 bar) to form a consistent wall thickness. The screw pushes against this pressure continuously – for weeks or months without stopping. With a standard gearbox, thrust bearing raceways spall, output shaft moves, and coupling misalignment follows – all within months. ZLYJ solves this by placing the hollow output shaft close to the load point, with a thrust bearing sized to the screw diameter, reducing bending moments and distributing the axial force properly.

Typical power range: 30 kW – 160 kW

3. Plastic Profile Extrusion (uPVC Windows / Door Frames / Trim)

Profile extrusion presents a different challenge. The axial loads are moderate, but they fluctuate unpredictably because complex die cross‑sections create uneven pressure distribution. A standard gearbox without a dedicated thrust bearing may survive a steady load, but fatigue failure comes quickly when the load cycles constantly. ZLYJ’s thrust bearing is designed for both steady and fluctuating axial loads, and the cast iron housing (on larger sizes) eliminates flexing under pressure.

Typical power range: 22 kW – 110 kW

4. Plastic Sheet Extrusion (PP / HIPS / ABS / PET)

The application: Single‑screw extruders feeding sheet dies for thermoforming, packaging, and automotive interiors.

Why axial thrust is high and how ZLYJ handles it: Sheet dies are wide and require very uniform melt distribution. Back pressure is typically 150‑250 bar, but the screw length is often longer (30‑36 L/D), increasing the axial force. The thrust bearing rating in ZLYJ series is matched to extruder screw diameter. For larger screws (120 mm and above), the bearing is sized to handle the higher torque and thrust simultaneously.

Typical power range: 75 kW – 200 kW

How to Choose the Correct ZLYJ Size

The torque number is the starting point. If you do not have it from your extruder supplier, here is a quick way to estimate it: Torque (Nm) ≈ (Screw diameter in mm)² × (Specific torque factor). A common factor is 1.5‑2.5 Nm/mm² for general extrusion.

Once you have the torque, match it to ZLYJ ratings using our selection table. The ZLYJ series covers from size 112 (approx. 8 kNm) up to size 450 (approx. 250 kNm output torque). The thrust rating is already built in – you do not need to calculate it separately.

However, the limiting factor is usually thermal power, not mechanical strength. Pelletizing and pipe lines run 24/7. If ambient temperature exceeds 35°C, order the gearbox with cooling coils (standard on larger ZLYJ sizes).

Finally, specify the hollow shaft diameter to match your extruder screw shank. ZLYJ hollow bores range from 70 mm to 200 mm.

Typical Motor Power Range for ZLYJ Series (4‑pole, 1500 r/min input)

Above figures are typical ranges. Actual selection requires torque verification.

Common Mistakes We Have Seen in Extruder Gearbox Selection

❌ Using a standard helical gearbox (R/F/K series) on a single‑screw extruder – The gearbox will fail. Not a matter of if, but when. The output bearings are not rated for axial thrust.

❌ Using a worm gearbox (RV/S series) “because it might handle axial force better” – Yes, worm gears have some axial capacity, but efficiency is only 50‑89%. For a 75 kW extruder running 24/7, the wasted energy is significant. Also, no dedicated thrust

bearing.

❌ Ignoring thermal power – A pelletizing line in a high-ambient-temperature environment needs cooling. Without it, the gearbox will overheat, oil viscosity drops, and bearings starve.

The right approach is straightforward: Use ZLYJ series. Specify cooling coils if ambient exceeds 35°C or if running 24/7. Provide the extruder screw diameter and expected back pressure – we will select the exact size.

A Real Example:Pelletizing Line Replaces Failed Standard Gearbox

Application: A recycling plant in a high-ambient-temperature environment runs a 90 mm single‑screw extruder for pelletizing post‑industrial waste. The original standard helical gearbox failed twice in 18 months – each time due to thrust bearing damage. The

original unit used tapered roller output bearings rated for radial load only. Under continuous 90 mm screw thrust, the bearing raceways showed spalling within 6 months. After the second failure, the plant replaced the unit with a ZLYJ 250, which uses a

dedicated thrust bearing package with axial capacity matched to the screw diameter.

Solution: HelmeDrive supplied a ZLYJ 250 gearbox with cooling coils and a hollow output shaft to directly accept the screw shank. Motor power: 90 kW, 4‑pole.

✅ Result: The gearbox has run for 26 months with no bearing issues. The customer reports stable oil temperature even in summer months. They have standardized ZLYJ on all three of their pelletizing lines.

�FAQ

Q1: Can I use a ZLYJ gearbox for a twin‑screw extruder?
A: No. ZLYJ is designed for single‑screw extruders only. Twin‑screw extruders require a different gearbox design (split torque, counter‑rotating outputs).

Q2: What is the difference between ZLYJ and a standard helical gearbox with an external thrust bearing?
A: An external thrust bearing (mounted on the extruder screw) is possible but adds cost, alignment complexity, and another maintenance point. ZLYJ integrates the thrust bearing inside the gearbox housing – simpler, more compact, and factory‑tested.

Q3: Does ZLYJ require special oil?
A: Standard mineral oil (ISO VG 220) is suitable for most conditions. For hot climates or 24/7 operation, synthetic oil (PAO VG 220) is recommended. The gearbox can be ordered pre‑filled with either.

Q4: How do I mount the screw into the hollow output shaft?
A: The extruder screw shank slides into the hollow bore. A locking device (shrink disk or key) secures it. ZLYJ series is supplied with a standard hollow bore – confirm the dimension when ordering.

Q5:  Can ZLYJ run in reverse?

A: Yes, ZLYJ gearboxes can operate in both directions. However, extruders normally run only forward. No special configuration is needed for reverse operation.

Get a ZLYJ Recommendation for Your Extruder Line

HelmeDrive supplies ZLYJ series gearboxes to extruder manufacturers and plastics processors worldwide. We provide full documentation: material certificates, test reports, and dimensional drawings.

Send us your extruder specifications:

◆ Screw diameter (mm)

◆ Motor power (kW) or output torque needed (Nm)

◆ Application (pelletizing / pipe / profile / sheet)

◆ Expected back pressure (bar) if known

◆ Ambient temperature at your site

We will reply with a matched ZLYJ size, cooling recommendation, and landed cost estimate.

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