As a rubber machinery salesman, I have sold several single screw extruders in both extrusion categories – hot feed and cold feed.  In each of my sale, the buyers had strong views and apprehensions on screw and barrel life, throughput and strained production capabilities, extrudate temperature, extrudate appearance, die head design and performance, etc (all of which I acknowledge are really important).

Strangely, the topic of gearbox usually concluded in few questions – “Which brands of gearbox do you offer and what is the SF?” or “If I select XYZ over ABC, will the delivery lead time of my extruder change?” or “Should I call you or the gearbox vendor for service during warranty period of gearbox?”

I do not recall a single discussion where the buyer asked me about gearbox technical details. Nor do I remember the buyer asking me to arrange for a separate technical discussion with extruder gearbox manufacturer to clarify their requirements.

Why do I think this aspect of joint discussion (if any) was relevant for me as a extruder supplier? Because the assembly of a rubber extruder “practically commences” only after the gearbox is ready at the extruder builder’s shop-floor. Unlike an internal mixer or a mixing mill or calender, buyers would not opt for buying the gearbox and main machinery separately. To any reputed machinery manufacturer, there is no bigger pet peeve than a customer buying different components from different vendors trying to save pennies. (But I digress here… do read Prof. Andreas Limper interview).

And yes, a gearbox is also the single-most-expensive-component on this popular rubber machinery.

I safely presume that this single argument reinforces the relevance of this post here – Why You Should Spend More Time Thinking about Gearbox for Rubber Extruders?

First things first – Why do your single screw extruders need gearboxes?

Extruder manufacturers prefer 1800/1500 RPM or 1200/1000 RPM motors (depending on your country of use) because they are economical, readily available and compact in size to mount on your extruder base. However, most rubber extruder screws during production run in the speed range of 4 rpm to 40 rpm.

Hence, the role of a gearbox or gear reducer here is to reduce the drive motor’s speed and, in turn, multiply the available torque from the motor in order to produce sufficient power to mix and push out your rubber compound.

As an individual and independent component, the key specification that defines the capacity and durability of your extruder gearbox is the power (HP/kW) rating along with its service factor (SF). Single screw extruder gearboxes are normally rated for power (HP/kW) or torque at a specific rpm based on common calculations and standards. This uniform standard allows you to compare gearboxes from different manufacturers.

Your extruder manufacturers follow these guidelines and select a model for different service factors and applications. A key question you should ask is whether, the SF considered by your manufacturer is optimal for your extrusion application or not.

Knowledge of AGMA (American Gear Manufacturers Association) recommendation will certainly help you to discuss better with extruder manufacturers, but experienced gearbox manufacturers can guide you even better. I have witnessed buyers in Asia been taken for a ride for their ignorance and offered lower specification gearboxes on their extruders to compete on cost.

When you compare gearboxes, always evaluate on calculated power. The formula for calculated power of a gearbox is:

                        Calculated Power = Quoted Power X Service Factor.

Typically, single screw rubber extruder gearbox has service factors of 1.5 or 1.75 for optimal operating capacity. For example, a 6 inch pin type cold feed extruder gearbox with a calculated rating of 367.5 kW would have a quoted rating of 210 kW with a 1.75 service factor.

The overall rating of a gearbox is based on the ratings of all its individual components. This includes the gear teeth design, gear hardness, shaft dimensions, bearing selection and sizes, housing design (thickness & rigidity), and thermal considerations. All these considerations are to ensure that your gearbox has sufficient support and capacity to effectively transmit the motor torque to the screw without significant distortion or failure.

Within the gearbox, the most important component (and most expensive) is it’s thrust bearing. You evaluate a thrust bearing based on its type and life (B-10 or L-10 rating).

The B-10 Life (sometimes called L-10 Life) of the thrust bearing is based on an engineering calculation that estimates the number of hours of operation at which 10% of the bearings are likely to fail. Additional rating adjustment factors are to be applied to the basic B-10 life based on application factors including how the bearing is mounted.

For example, a thrust bearing that is mounted between two radial bearings is more likely to have precise thrust bearing alignment, and will therefore have a higher rating adjustment factor.

Here are 6 other key criteria of a gearbox evaluation, which you should know

1. Gear Design, Hardness & its Construction – Each of the individual gears that go into your gearbox assembly are rated for power or torque based on their strength and durability ratings. The calculations would be according to industry standard AGMA rating systems. Factors include the gear tooth pitch, center distance, material and hardness.
2. Gear Shafts – The shafts must be designed to transmit the full power and torque capacity of the gears. The length and diameter of these shafts is decisive and must match the ability to transmit this torque without excessive deflection, fatigue and failure. The diameter of the input shaft must be adequate to properly support sheaves (in the case of belt driven models) or a coupling. The output shafts must be properly designed to handle the correct range of screw shanks that will be inserted. Adequate access to the drive keys is beneficial when they become worn or damaged and need to be replaced.
3. Radial Bearings and Seals – The radial bearings support the rotational forces of the gear shafts and must be designed to handle the load forces and speeds effectively. The dynamic load capacity of these radial bearings must also be considered when evaluating the design and durability of the gearbox. Radial bearings must also be properly lubricated and sealed.
4. Gearbox Housing Design and Construction – Cast Iron is the cost-effective material of choice for most manufacturers. Traditionally, CI gearboxes are made in two pieces, split either horizontally or vertically. Newer designs have the gearbox housing cast as one piece to reduce any potential leakages.
5. Thrust Bearing – Thrust bearing isolates the backward forces from the screw. The larger the screw and/or the higher the back pressure, the greater the backward thrust forces. There are three basic types of thrust bearings – cylindrical, spherical and tapered.
6. Serviceability – When you select a gearbox, you should give prime importance to the availability to affordable parts and service. It is best to select rubber extruder suppliers who purchase their gearboxes from proven and reputable manufacturers that specialize only in gearboxes for better serviceability.

A Note of Caution:

If you plan to replace your old gearbox or comparing one, take note of below developments.

An old gearbox manufactured around through-hardened process and shaved gears technology has shafts, bearings and housings designed accordingly. Replacing new hardened gears with a higher HP capacity, does not automatically guarantee the gearbox rating to increase, if you do not replace the assembly with stronger shafts, bearings and housing.

Gear manufacturing technology today has changed and consists of carburized and ground gears. These gears are capable of delivering much more power in its smaller size. When old gear designs are constructed using the new materials and process, the power calculations yield much higher gear tooth ratings. But if the rest of the design is unchanged, and the same bearings, shafts, and housings are used, the total gearbox rating cannot simply be based on the new higher gear rating alone.

The higher torque could never be applied to the original sized input shaft without causing bending or twisting. The bearings and/or shafts would be overloaded with the higher forces, and the housing would probably not have sufficient strength to resist significant distortion. Reputed gearbox rebuilders will guide you well.

Summarizing, as an extruder buyer, you need to pay extra attention to the design and manufacture of the gearbox when evaluating and selecting a single screw extruder. 

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