🔥 Which Bearing Runs the HOTTEST in GE, Siemens & MHI Turbomachinery?

🔥 Which Bearing Runs the HOTTEST in GE, Siemens & MHI Turbomachinery?

Let me ask you a simple question…

👉 In a GE Frame 5, 7, or 9 gas turbine…
👉 Or a Siemens / MHI turbomachinery train…

Which bearing do you think runs the hottest?

Most people answer quickly…
❌ “Thrust bearing!”
❌ “Generator side!”

But the real answer is more nuanced—and much more interesting. In large heavy-duty gas turbines such as GE Frame 5, 7, and 9, and in comparable Siemens and MHI machines, the bearing that is most often the hottest journal bearing is the turbine-side bearing, commonly referred to in GE-type numbering as Bearing No. 2. The practical reason is simple: that bearing sits closest to the hot section / exhaust frame / turbine end, so it is exposed to a much harsher thermal environment than the compressor-end bearing. GE-related technical material repeatedly shows special cooling and sealing provisions around this bearing area, and GE training material explicitly highlights #2 bearing and exhaust frame cooling as a distinct concern. (HPC Technical Services)

That does not mean the answer is always “No. 2” in every machine and every operating mode. The correct engineering answer is: the hottest bearing depends on whether you are talking about the hottest journal bearing or the hottest overall bearing metal location, including thrust pads. In many gas turbines, the turbine-end journal bearing is the hottest journal bearing; but in many steam turbines and some mechanical-drive trains, the thrust bearing can be the hottest overall bearing metal temperature location because it carries the axial load of the rotor. General gas-turbine references also separate these functions: journal bearings support radial load and rotor position, while thrust bearingsabsorb axial thrust. (netl.doe.gov)

The practical answer by OEM family

1) GE Frame 5, 7, and 9

For the classic GE heavy-duty frame machines, the most likely hottest journal bearing is Bearing No. 2, the turbine-end bearing. Why?

First, it is physically close to the turbine/exhaust region, where surrounding structure temperatures are much higher than at the compressor inlet end. GE-related literature and training references specifically call out the exhaust frame and #2 bearing cooling, and plant troubleshooting discussions on Frame 5 and Frame 9E repeatedly report Bearing No. 2 as the bearing that trends hottest or alarms first. (HPC Technical Services)

Second, this area depends heavily on cooling/sealing air as well as lubrication. Public GE-oriented material notes that cooling and sealing air for the turbine bearings is taken from compressor extraction air, and one ASME-indexed result specifically states that Bearing No. 2 must be cooled and purged by the proper amount of cooling air. That is a major clue: the OEM system design itself treats this bearing as especially thermally sensitive. (www.slideshare.net)

Third, the bearing housing arrangement itself is more elaborate around that zone. A Frame 7EA bearing-housing description shows a triple-pipe arrangement providing separate air passages to the bearing, which is exactly the kind of architecture you expect where thermal management is critical. (Scribd)

So for GE Frame 5 / 7 / 9, if someone asks you in the field, “Which journal bearing usually runs hottest?” the safest expert answer is:

Usually the turbine-end journal bearing, commonly Bearing No. 2.

But if the question is, “Which bearing metal can be highest overall?” then the answer may shift to the thrust bearing pads, depending on thrust load, alignment, oil flow, and pad design. (Control.com)

2) Siemens turbomachinery

For Siemens gas turbines, the same general thermal tendency usually applies: the turbine-end journal bearing is typically under the highest thermal stress among the radial bearings because it is nearest the hot gas / exhaust side, while the compressor-end bearing lives in a cooler environment. Even where public Siemens gas-turbine product pages do not spell out “hottest bearing,” the machine architecture and hot-section arrangement support the same logic seen across heavy-duty gas turbines. (Siemens Energy)

For Siemens steam turbines, however, the picture often shifts. Siemens literature explicitly says the use of balancing pistons minimizes thrust and allows smaller axial bearings. That wording matters because it implies the axial bearing is a major design concern, and OEM effort is spent reducing the load it sees. In steam turbines, especially high-output or process-drive units, the thrust bearing is often the most thermally critical bearing location because axial load concentration, oil-film shear, and pad load distribution can push pad metal temperatures above those of the radial bearings.

So for Siemens, a practical summary is:

• Gas turbines: hottest journal bearing is usually the turbine-end journal bearing.
• Steam turbines / some process-drive machines: hottest overall bearing metal location is often the thrust bearing. (Siemens Energy)

3) MHI turbomachinery

MHI public technical papers are actually very helpful here. In one MHI steam-turbine paper, they state directly that the thrust bearing, under high-speed and high-load operation, had bearing metal temperature increased, enough to threaten reliability margin. MHI then describes design changes specifically intended to lower thrust pad temperature, including direct lubrication, improved leveling, chromium-copper pad backing, and offset pivots. (Mitsubishi Heavy Industries, Ltd.)

That is extremely revealing. When an OEM spends design effort on direct lubrication to reduce thrust pad temperature rise, it tells you that in those MHI steam-turbine applications, the thrust bearing is often the thermally limiting bearing. (Mitsubishi Heavy Industries, Ltd.)

MHI gas-turbine material also shows the same pattern seen in other heavy-duty units: the turbine side is thermally more severe. In one MHI gas-turbine driver example, published data compare active-side thrust bearing metal temperature, inactive-side thrust bearing metal temperature, and journal bearing temperatures, again showing how closely OEMs monitor the thrust location when thermal margin matters. (Mitsubishi Heavy Industries, Ltd.)

So for MHI, the best summary is:

• Gas turbines: hottest journal bearing is usually the turbine-end journal bearing.
• Steam turbines / high-axial-load mechanical-drive turbines: the thrust bearing is often the hottest and most critical from a metal-temperature standpoint. (Mitsubishi Heavy Industries, Ltd.)

Why these bearings run hottest

A) Proximity to the hot section

This is the biggest reason for turbine-end journal bearings. The turbine-side bearing housing is close to the exhaust frame, hot casings, wheelspace, and hot surrounding metal. Even with insulation, seals, purge air, and oil cooling, more radiant and conducted heat reaches that zone than the compressor inlet end. GE-oriented sources highlighting dedicated #2 bearing cooling support this directly. (HPC Technical Services)

B) Dependence on cooling and sealing air

On heavy-duty gas turbines, bearing temperature is not only an oil issue. It is also a cooling-air and sealing-air issue. If extraction-air pressure drops, passages foul, vents are blocked, or purge/cooling flows are mis-set, the turbine-end bearing can heat up quickly. This is why operators often see Bearing No. 2 become the first abnormal temperature trend in GE-type fleets. (Control.com)

C) Axial load concentration on thrust bearings

For thrust bearings, the reason is different. A thrust bearing converts rotor axial force into hydrodynamic film pressure across loaded pads. That means localized oil-film shear, pad load concentration, and pad-metal temperature rise, especially at high load, high speed, marginal oil flow, misalignment, or poor pad leveling. MHI’s direct discussion of thrust-bearing temperature rise is an excellent OEM confirmation of this mechanism. (Mitsubishi Heavy Industries, Ltd.)

D) Oil-film shear and scavenging quality

Even with correct load, poor oil feed temperature, insufficient flow, aeration, drain restriction, or scavenge problems can make the hottest bearing hotter still. GE operating literature stresses monitoring bearing metal and bearing draintemperatures as key health indicators. (gevernova.com)

My practical ranking

If you want a field-style ranking for large utility/industrial turbomachinery, this is the most defensible simplified rule:

For heavy-duty gas turbines (GE Frame 5/7/9, Siemens heavy-duty GTs, MHI large GTs):
Hottest journal bearing = usually the turbine-end bearing
(on GE-type numbering, commonly #2 bearing) (HPC Technical Services)

For steam turbines and some mechanical-drive turbomachinery:
Hottest overall bearing metal temperature = often the thrust bearing

Important caution

Do not turn this into a blind rule. On your actual machine, the hottest bearing may shift because of:

• bearing numbering convention
• single-shaft vs two-shaft arrangement
• generator-coupled vs compressor-drive layout
• load and ambient changes
• oil header temperature
• seal-air / cooling-air performance
• pad design and pivot geometry
• alignment and rotor position
• probe or RTD location

That is why the best engineering language is “usually,” “commonly,” or “most often,” not “always.” (gevernova.com)

Final conclusion

If you ask me for the cleanest expert answer:

In GE Frame 5, 7, and 9 gas turbines, and generally in comparable Siemens and MHI heavy-duty gas turbines, the hottest journal bearing is usually the turbine-end bearing because it is closest to the hot section and depends strongly on cooling/sealing air effectiveness. In GE-type machines this is very often Bearing No. 2. (HPC Technical Services)

But when thrust bearings are included in the comparison, especially in Siemens and MHI steam-turbine or high-axial-load process-drive applications, the thrust bearing can be the hottest overall bearing metal location because axial load and pad heating dominate.

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