High-Speed-Gearboxes ISAR GETRIEBETECHNIK GMBH & CO. KG

High-Speed-Gearboxes 
- an Overview

A website of 
ISAR GETRIEBETECHNIK GMBH & CO. KG

What is High Speed?

The terms „High-Speed Gearbox“, „High-speed gear reducers“, „High Speed Reducer“/“High Speed Increaser“, „Turbo Gearbox“, and „Turbo Reducer“ are synonyms and do not have exact differentiation, but are usually used according to their field of operation („Turbo“ for turbine applications etc.)
The term „High Speed“ is many times referred to as a high value of RPM, thus a high number of revolutions per time slice. So, some call a gearbox a „high-speed gearbox“ (or a synonym) if it is able to run more than 3000 rpm, for instance. But from the technical point of view the circumferential velocity is a much more precise expression. The speed needs to be related to size!
While a small gearbox with a pinion diameter of 5 mm and speed of 50'000 rpm has a circumferential velocity of 53 m/s, a turbine shaft with a diameter of 1 m and 3000 rpm has a circumferential velocity of 157 m/s. And the latter necessitates much more technical efforts in order to get a stable system. Therefore, a sensible technical description of speeds of high-speed drives always takes into account the size or power class.

Main parameters for stability of a high speed system (as a gearbox) are:
· Unbalance quality (proportional to speed)
· Centrifugal forces (proportional to size and squared speed)
· Circumferential speed (proportional to size and speed)

In order to acknowledge the over-proportional influence of speed on a dynamic system, this should be considered when defining high-speed drives. It is proposed to define the following limit for „real“ high speed drives:

P ∙ n³ ≥ 1014
where Power is P [kW], Speed n [rpm]

→ limit is e. g. 100 kW @ 10.000 rpm

This limit can be depicted as follows in a double-logarithmic diagram:


SpeedOverPower[High-Speed-Gearboxes]

Why Do I Need High-Speed-Gearboxes?

Considering the power density of an electric engine, there are following basic relationships:

- Torque of an engine is dependent on length (proportional) and diameter (proportional by square)
- Power is product of torque and speed

P [kW] = M [Nm] ∙ ω [sec-1]
(where P is power, M is torque, and ωω is angular speed)

That means:
→ The higher the speed, the higher the power – at same size!

Or vice versa:
→ The higher the speed, the lower the weight/cost/space - at same power!

· Less material
· Less weight
· Less costs (up to a certain point)

(These relations are a bit more complicated with a combustion engine, but the principle is still valid.)
Therefore, the trend towards steadily increasing drive speeds (especially for electric drives) can be understood.

For adapting the speed of an high-speed drive a high-speed reducer is finally necessary which delivers the power at an applicable speed. In most applications with moderate operational speeds a combination of high-speed drive and high-speed reducer is superior to a direct drive regarding power density or power/weight ratio. Such applications are vehicle drives, prop drives, drives for industry, robots, handling.

What are the Main Challenges about High-Speed-Gearboxes?

When designing a fast turning rotor it must be taken into account that different aspects arise than with low-speed rotors:

1. Rotor Stability
· Centrifugal forces occur with increasing speed (proportional by spare speed!) and may lead to bursting which is to prevent in any case.
· Setting effects may come into force and deteriorate unbalance which leads to undefined bearing loads.

2. Noise and Vibration Harshness (NVH)
A special difficulty for electric vehicles is the lack of the predominating sound of a combustion engine. So, the sound of other components as a gearbox comes to the fore:
· High-speed gear-mesh produces inconvenient sound for human sense of hearing.
· Ultrasound is usually too high to be in the range of operating conditions. Even if, the lower speed stages run then at high meshing frequencies and uncomfortable sound.

3. Rotor Dynamics
Eigenfrequencies of high-speed rotor are strongly influenced by applied masses. A common set-up is a heavy test equipment which is fixed on a high-speed shaft. The following example shows an intended set-up of a shaft in an high-speed application. The gap between intention and reality is factor 2.


Required speed: 25.000 rpm[High-Speed-Gearboxes]
Required speed: 25.000 rpm
Eigenfrequency: 11.900 rpm[High-Speed-Gearboxes]
Eigenfrequency: 11.900 rpm


4. Bearings
A critical component of stable rotors is always the enduringly stable bearings. But:
· Among standard rolling bearings only spindle bearings are appropriate for high-speed applications. Those have a low load capacity.
· Sliding bearings are made for high speed and loads. They lead to high costs with lots of oil supply and produce high power losses.

5. Lubrication
Because of the high sliding speed the lubrication situation in a high speed gear mesh is usually a full EHD contact which is preventing wear. But a critical parameter is high temperature in sliding contact (gears as well as bearings; flash temperature) which promotes scuffing damages. When dimensioning the lubrication supply of a high-speed system it needs to be considered:
· Lubrication of gear mesh may be prevented by centrifugal forces
· Too much oil in gear mesh may cause high power losses and, thus, high temperature.
· Not enough oil in gear mesh may not be able to transport heat from contact to environment and cause high temperature, accordingly.

6. Sealings
Contact sealing materials are limited to circumferential speed of some 40 m/s. Above, non-contact seals (as labyrinth seals) or face seals need to be used (which is common for at least the high-speed shaft of a high-speed gearbox).

7. Power losses
At high speeds the no-load losses predominate over load-dependent losses at high speeds (while at lower speeds the load dependent losses of the gear mesh are prevailing other power loss components by far), see picture below. For a efficient operation at high speeds ventilation, splashing, and oil squeezing must be addressed.


LossOverSpeed[High-Speed-Gearboxes]

How to Address
these Issues?

1. Rotor Stability
· High-speed rotors need to be run at over-speeds in safe test environment prior to operation in order to prevent bursting during later operation in less protected applications.
· The main line of action is: balancing, balancing, balancing

2. Noise and Vibration Harshness (NVH)
· An optimization of NVH behaviour can only be applied to determined operation loads. Therefore, operating conditions need to be prioritized or weighted.
· When using high-quality gears with elaborate flank modifications for defined operation conditions (load and speed) significant reduction of of noise and vibration can be achieved.
· Occasionally, active or passive damping layers need to be applied in order to reach given NVH levels.

3. Rotor Dynamics
For rotor dynamics at high speeds the correct mass-stiffness-relation is crucial.
· With increasing speed the design of light-weight structures is more and more important.
· Basic parameter are stiff rotor structures with largest areas of moment inertia in sections with highest bending stress.
· Short distances from mass centers to bearings (especially for overhanging masses) contribute to advantageous rotor dynamics behaviour.


High-Speed Shaft-Mass or Opt[High-Speed-Gearboxes]


4. Bearings
· Rolling contact bearings with sophisticated designs and materials allow much more extensive laod and speed parameters than standard bearings. Such types of bearings are common in aircraft applications, they are rather complex and expensive, though.
· A design layout which helps to control forces onto high-speed bearings is a planetary or star design. With such a layout the radial forces onto the sun shaft compensate themselves (nominally), see sketch. The bearings only have to guide the sun shaft or take over axial forces. Such layout is (almost) mandatory at very high speeds. The depicted gearbox runs at up to 160.000 rpm at 85 kW.


Gearboxes01[High-Speed-Gearboxes]
Gearboxes02[High-Speed-Gearboxes]


5. Lubrication
High centrifugal forces resulting from high speed of shafts need to be overcome by lube oil in order to get to the right place at bearings and gears. That leads to pressurized oil feed in most cases where enough pressure generates sufficient jet velocity to make the lube oil reaching its aim safely. This is especially the right method of lubrication if a wide range of operating speeds needs to be covered. In best case the oil pump is controllable and can adjust oil volume to need according to operating conditions in order to minimize power losses.
· Otherwise – if geometry and space is appropriate – centrifugal forces can even be useful help and make lubrication possible from inside a shaft. The turning of the shaft transports the lube oil through outlets to defined places. In this case a certain minimum speed is always necessary.

6. Sealings
The high speed shaft tends to be short but massive for reasons of rotor dynamics. That mostly leads to a sealing diameter which is beyond the speed limit of contact seals. Possible alternatives are
· Labyrinth seals – those are statically leaking, thus, must be located above a possible oil fluid level. Depending on design and number of labyrinth chambers the axial space is larger than for simple contact seals. They produce (almost) no power loss and, therefore, do not induce heat input into the sealing diameter which is a very positive aspect.
· Floating ring seals which can be purchased as preconfigured (though complex) units. Disadvantageous is besides costs is their large axial space.

Basically, the aim of development and research of sealing manufacturers is to increase speed of contact seals steadily. A quantum leap is not foreseeable in the next future.

7. Power losses
Measures against increasing power losses with higher speeds (circumferential/angular) can be following actions:
· Low oil viscosity
· Moderate oil volume, up to minimum quantity lubrication
· Ample backlash and tip clearance in order to minimize oil squeeze in gear mesh

When defining the lubrication quantity the challenging issue is to find the right compromise between moderate power losses and enough heat transfer out of the hot spots of the gearbox.

Contact /
Imprint

ISAR GETRIEBETECHNIK GMBH & CO. KG
Muenchener Str. 101 - Building 38
D - 85737 Ismaning
Phone: +49 - 89 - 127 66 88 -0
Email: contact@isar-gears.com
Official website [EN]: www.isar-gears.com
Commercial Registry: 
Amtsgericht München, HRA 92707
Sales Tax Identification Number: 
DE261471260
General Partner: 
Isar.Pro GmbH, Amtsgericht München,
HRB 256168
General Manager: 
Dr.-Ing. Albert J. Wimmer

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