Steam does one job on a gearbox housing and it is a narrow one: it softens the oily film sitting on the outside so the film can be lifted off with a cloth. Grease that has hardened in a casting seam, a bolt ring or a paint drip behaves differently from fresh splash, and that difference, not the machine brand, decides whether the cover comes back clean or the job stops there.
Fresh splash and baked film are two different problems
Fresh splash is oil that left a seal, a coupling or a filling point recently and has not yet collected anything. It responds quickly to heat, and a short dwell is usually enough before wiping. Baked film is the same oil after it has sat through hot running cycles, grabbing dust, swarf fines and floor debris into a skin that bonds to the paint. Heat softens that skin but does not release it on its own, so the plan needs dwell plus mechanical work by hand, and sometimes a repeat pass on the worst panels.
Grease that has migrated into a seam is the hard case. Once material has worked its way into a casting joint or under a gasket edge, cleaning the face above it can leave the deposit in place and simply move it. Ask a supplier which of those three states they expect to meet on your equipment before discussing any heavy-duty steam cleaning machine for the area.
What heat actually changes on a gear cover
Heat lowers the grip of the oil on the surface and it speeds up the softening of the trapped particles inside the film. It does not dissolve anything, and it does not make the deposit leave the metal. That is why an application built on volume of water fails here: the water is what carries the loosened material into seams and under plug threads, and the heat is what did the useful work. A method that puts more heat onto the surface and less liquid across it is the one worth testing near a gear case.
That distinction also sets the expectation for the finish. A cover can look clean at the lamp and still hold oily residue in a texture or a fillet. Decide before the work what counts as clean, and check it on a small panel first rather than after the whole unit has been worked on.
The parts of the unit that must stay dry
Every gear unit carries openings that were never meant to see spray. A vent or breather equalises pressure and is a direct path to the oil inside. A sight glass sits in a machined recess where water can sit after the jet stops. Fill and drain plugs thread into bosses, and any moisture held in the thread damages the seal the moment the unit is restarted. Bearing ends, shaft seals, the motor fan cowl and the terminal box are treated the same way: shielded, worked around, and never jetted at.
The nameplate and any inspection marks belong on the protected list as well. A cleaned unit with an unreadable plate or a wiped maintenance sticker has lost information somebody needs in the next teardown. Write the protected items down before the crew arrives, with a named person responsible for shielding them and for confirming they are clear again afterwards.
Run-off and the residue you are moving off the unit
The material leaving a gear cover is oil, abrasive fines and whatever the film collected from the floor. On a pedestal unit it runs to the base and pools there; under a hollow-shaft or shaft-mounted drive it can travel along the shaft into the machinery it is bolted to. Where the unit sits above a pit, a belt or a cable tray, gravity decides the route and the cleaning plan has to decide it first. Catch trays, a defined working direction and a collected waste route are part of the method, not an addition to it.
Ask what the receiving drain is allowed to take. Mixed oil and fine solids are a waste question for your site, and the answer changes whether an outdoor wash area is usable or whether the work belongs in a bunded space with recovery.
Where steam is the wrong tool for this cover
Do not put steam near a gear unit that is already weeping at a seal, because cleaning will not be the cause of the next leak and it will be blamed for it. Do not use it where a film returns within days of every clean, since that points to a lubrication fault rather than to a cleaning interval. Do not use it before an inspection that has to read a crack, a witness mark or a repair, and do not treat a hot, still-running unit as a cleaning job.
Some covers are better solved by a degreasing product and a hand wipe with no spray at all, particularly on units that sit in an electrical enclosure or above sensitive drives. Where that is the honest answer, the supplier who says so is the one worth keeping.
Asking the supplier the right question
Send photographs of the worst panel and the worst seam, name the deposit state you believe you have, and list the openings and plates that must stay dry. Then ask for a written answer to a bounded trial: one cover, one defined cleaning window, the protected items named, the run-off route stated, and a written record of what stayed behind.
Ask how that trial compares with work they have done on comparable exteriors, and ask for the reference in writing. If the experience lives on an industrial steam cleaner page rather than in a service record, ask what was actually cleaned and what was left behind. A supplier comfortable describing failures is describing real work.
Writing the trial into a specification
Write the scope as surfaces, not as a machine. Name the cover, the fins, the bolt rings, the base and the coupling guard as cleanable; name the vents, sight glasses, plugs, bearing ends, seals, plates and electrical items as excluded. State the acceptance check for each cleanable surface, and state who signs the unit back into service once it is dry.
Attach the photographs, the protected list and the run-off route to the enquiry. That single document is what lets two quotations be compared at all; without it, both bidders are describing different work. Where a fixed unit suits the area better than a mobile one, the same document is what a supplier needs before pointing you at Fixed Steam Cleaning Machine PowerDe HW02.
