Function: The crosshead guide shoe (slipper) is fitted to the crosshead of a large two-stroke engine and runs on the vertical guide faces in the engine frame. Its function is to guide the crosshead (and hence the piston rod and piston) in a straight vertical line, preventing the crosshead from rotating and keeping the piston central in the cylinder. It absorbs the side thrust (the horizontal component of the connecting-rod force) and transmits it to the engine frame, preventing the piston from being forced against the liner. It also maintains the correct alignment of the piston rod and crosshead.

Lubrication: The guide shoe is lubricated by the engine's lubricating oil system. Oil is fed to the guide shoe/ guide faces, either by a dedicated lubrication feed (from the main lube oil system, sometimes via a booster/ separate pump) or by oil splash/ spray. In many designs, oil is supplied to the crosshead and guide shoe through the crosshead pin/ oil ways, and the guide faces are lubricated by oil that is distributed to the guide shoe. The oil forms a film between the shoe and the guide, reducing friction and wear.

Maintenance:

• Regular inspection of the guide shoe and guide faces for wear, scoring, pitting and correct clearance.
• Measurement of the guide shoe clearance (feeler gauge) and adjustment (by shims) to the maker's specification.
• Checking the lubrication supply to the guide shoe (oil flow/ pressure) and keeping the oil ways clear.
• Renewal of the guide shoe (white-metal/ bearing surface) when worn.
• Checking the guide faces for wear/ damage and the alignment of the crosshead.

Effects of improper maintenance:

• If the guide shoe clearance is too large or the shoe is worn, the crosshead can move sideways, causing the piston to be forced against the liner (scuffing/ scoring of the liner and piston), excessive side thrust, and increased wear.
• If the clearance is too small, the shoe can bind/ seize on the guide, causing overheating, scoring and possible seizure.
• If the lubrication fails, the guide shoe and guide faces overheat, wear rapidly, and may seize, causing damage to the crosshead, piston rod and engine.
• Misalignment of the crosshead/ guide shoe causes uneven wear, increased vibration, and damage to the piston, liner and bearings.
• Improper maintenance can lead to catastrophic engine damage (seizure, broken piston rod, damaged liner).