A-Frame and Column Design in Two-Stroke Engines

The A-frame is the middle structural tier of a large two-stroke crosshead marine diesel engine, sitting between the bedplate below and the cylinder frame above, carrying the crosshead guides, enclosing the crankcase, and transmitting cylinder firing load down to the bedplate through the stay bolts or tie rods that clamp the three tiers into one unit. MAN and Everllence call it the frame box; WinGD calls it the column.

The three tiers and what each one carries

A large two-stroke crosshead engine is a stack of three structural tiers clamped together by pre-tensioned fasteners, and each tier has a different job. A medium-speed four-stroke engine has no equivalent middle tier, because it has no crosshead and its running gear sits in a single monobloc frame. The bedplate at the bottom carries the crankshaft and delivers all engine loads into the ship’s double bottom. The frame box in the middle carries the crosshead guides and closes the crankcase. The cylinder frame at the top carries the cylinder liners, the scavenge air space and the cylinder covers.

Everllence describes the bedplate of the G95-50ME-C10.7/.5 as welded longitudinal girders and welded cross girders with cast steel bearing supports. The frame box above it is welded. The cylinder frame is offered two ways: nodular cast iron, or a welded design with an integrated scavenge air receiver, which is the pressure side of the turbocharging arrangement.

That division matters for anyone tracing a load or planning a repair. Vertical firing load is carried in tension by the stay bolts and in compression by the tier faces they clamp. Lateral load from the running gear enters the structure at the guide faces on the frame box, not at the bedplate and not at the cylinder frame. Sea-induced hull deflection enters at the seating, below all three.

Manufacturer terminology, and why reading across two builders’ manuals goes wrong

The same component carries different names and different fastener counts at the two builders that dominate current two-stroke newbuilding, so a figure lifted from one manual does not transfer to the other. The third licensor family, Mitsubishi UEC , uses its own vocabulary again. The table below reconciles the vocabulary as each builder’s own documentation uses it.

Source: Everllence ME Engine Description, G95-50ME-C10.7/.5, document 199 07 85-8.4, 23 June 2025; WinGD X92-B Operation Manual, Issue 002, November 2021.

Two of these rows are the ones that get mixed up in secondary sources. A “tie rod” count taken from WinGD literature and applied to an Everllence engine doubles the real number, and a “stay bolt” figure applied to a WinGD engine halves it.

The load path terminates at the cylinder frame, not the cylinder cover

The stay bolts clamp three tiers, and the cylinder cover is held by a separate fastener system. Everllence states that stay bolts tighten together bedplate, frame box and cylinder frame, and separately that the cylinder cover is attached to the cylinder frame with studs and nuts tightened with hydraulic jacks.

The MAN B&W S50MC-C instruction book gives the lower termination directly:

The bedplate, the framebox, and the cylinder frame … are tightened together to form one unit by means of stay bolts, the bottom of which is screwed into the bedplate.

So the lower end is threaded into the bedplate rather than held by a nut in a bedplate recess, and the upper end stops at the cylinder frame. An account that routes the tie rod from the cylinder cover to the bedplate has merged two independent joints into one and will mislocate every reaction in the stack.

WinGD describes the same three-tier clamp with a different count and one extra safety feature: the tie rods keep the cylinder block, column and bedplate together at four locations around each cylinder, and if a tie rod breaks in the bottom area, a special device makes sure that the nut of the tie rod does not fall into the crankcase.

Tensioning values are engine-model specific. Rod diameter, free length, hydraulic tensioning pressure and the tensioning sequence come from the engine builder’s maintenance manual and the tensioning tool data sheet for that engine, and no generic figure substitutes for them.

Crosshead guides are welded onto the frame box

On current Everllence and MAN B&W two-strokes the guide faces are welded to the frame box, not bolted to it. The ME Engine Description for the G95-50ME-C10.7/.5 and the G60ME-C9.5 project guide use the same words: crosshead guides are welded onto the frame box.

This decides what a guide-face defect costs. A bolted rail is a spare part. A welded guide face is part of the frame box, so restoring geometry is a machining or weld-repair job carried out on the engine, with alignment re-verified afterwards against the works records.

The moving half of the pair is where the sacrificial material sits. Everllence describes a forged steel crosshead provided with cast steel guide shoes with white metal on the running surface, of the low-friction type, running with wide-pad crosshead bearings. The telescopic pipe for oil inlet and the pipe for oil outlet are mounted on the guide shoes, so crosshead lubricating oil is routed through the shoe rather than through the guide face.

Lateral force at the guide face, and where it peaks in the cycle

Lateral force at the guide is the piston force multiplied by the tangent of the connecting rod inclination, so it is zero at top dead centre and zero at bottom dead centre. Rod inclination is at its maximum where the crank is perpendicular to the connecting rod, roughly mid-stroke. Peak guide load therefore occurs after top dead centre, at the crossing point where the falling gas pressure and the rising rod angle produce their largest product.

The WinGD Marine Installation Manual for the X92DF sets out the force system in section 6.2, “External lateral forces and moments”, with force diagram SM-0514 defining the gas force FG, the piston force FP, the connecting rod force FC, the lateral force FL, and the radial and tangential components FR and FT:

The forces between the piston and the connecting rod reaction cause a lateral force to act on the crosshead guide rails. The lateral forces at the guide rails are transmitted to the engine block and to the foundation.

That last clause is the design consequence. Guide load does not stop at the frame box. It passes into the seating, which is why athwartship restraint at the bedplate is a separate design item from vertical support.

A widely repeated claim places peak side thrust at top dead centre on the reasoning that cylinder pressure peaks there. It does not follow. At top dead centre the rod is vertical and the tangent of its inclination is zero, so the lateral component of even the largest piston force is zero.

Lateral vibration: H-type and X-type modes

A two-stroke engine’s lateral vibration appears in two mode shapes, and WinGD publishes a moment value for each. Section 6.2.1 of the Marine Installation Manual identifies H-type and X-type modes, and the corresponding H-type and X-type moment values are given per engine so the installation designer can assess the response of the engine and its seating together.

The distinction is structural rather than nominal. The two modes load the seating differently, so a single lumped “lateral moment” figure is not enough to size the restraint arrangement, and neither mode is captured by the vertical firing-load analysis that sizes the stay bolts.

Welded construction above and below the column

The frame box is welded, and casting survives on the tiers either side of it, which contradicts the common description of casting as a legacy two-stroke technique. Everllence offers the cylinder frame in nodular cast iron or as a welded design with an integrated scavenge air receiver, and specifies cast steel bearing supports in the welded bedplate. The MAN B&W G60ME-C9.5 project guide states plainly that the frame box is of welded design and the cylinder frame is cast.

Casting on these engines is also current engineering rather than inherited tooling. MAN publication MUN2018-04-25, covering the G95ME-C10.5, describes a new cylinder frame design developed using topology optimisation in combination with casting simulations.

The practical reading is that construction method follows the load case. The column takes concentrated guide reactions along its height and suits a fabricated box. The cylinder frame and the bearing supports take distributed, geometrically complex loads and suit a casting.

Engine seating: holding-down bolts, side chocks and end chocks

The seating restrains the engine in three directions with three different devices, and only the vertical one is the holding-down bolt. The MAN B&W S50MC-C instruction book describes the arrangement in full:

The bedplate of the engine is positioned on either epoxy or cast iron supporting chocks and bolted to the engine seating in the ship by means of long holding-down bolts, which are equipped with distance tubes of cast iron … spherical washers and nuts with spherical contact face on the lower end of the bolts.

Athwartship restraint is by side chocks. The same instruction book specifies side chocks fitted on both sides in way of a bedplate cross-girder, with side-chock liners tapered 1:100 and fitted from the aft end at both sides.

Fore-and-aft restraint is by end chocks, and there are two of them. The engine is secured fore and aft by one end chock with one end-chock bolt with spherical washer at the aft end of each of the two longitudinal girders of the bedplate, the end-chock liners also tapered 1:100 but fitted from above rather than from aft.

Everllence describes the vertical fastener as a long elastic holding-down bolt tightened with hydraulic tools, and adds one geometry note that decides the machining of the bedplate underside: for engines mounted on epoxy chocks, the bedplate is made without taper. A cast iron chock arrangement needs the tapered seat; an epoxy pour does not, because the resin takes up the geometry itself.

The seating arrangement in full, across chock materials, bolt pre-tension and the re-chocking case, is covered in engine chocking and holding-down arrangements . WinGD treats the seating as ship structure rather than engine structure. The Marine Installation Manual for the X92DF, sections 3.5 and 3.6, has the seating integral with the double-bottom structure, with the bedplate carrying tapped holes for jacking screws and drilled holes for holding-down bolts. The detailed procedures sit in WinGD Fitting Instruction DG 9710 for engine seating and foundation and DG 9709 for alignment.

Epoxy chocking: the properties that matter

Epoxy chocking resin for main-engine seating sits near 3.6 to 3.7 GPa in compressive modulus, roughly one fiftieth of steel, and that compliance is the point: the resin conforms to the as-built geometry instead of forcing a machined fit. The table gives the published values for the grade most often specified for main-engine chocks.

Source: ITW Performance Polymers, Chockfast Orange (PR 610 TCF), Technical Bulletin 659H, June 2021.

Cure time is temperature dependent and is a real constraint on a yard schedule: 18 hours above 21 degrees Celsius, 24 hours at 19 to 21 degrees, and 48 hours at 13 to 18 degrees. A winter fit-out in an unheated engine room is a two-day hold on the critical path, not an overnight one.

The grade family is narrow. The ITW comparison sheet, document 840D revision 03/2024, gives Chockfast Black at 5.52 GPa and Chockfast Gray at 3.59 GPa. Any chocking modulus quoted well outside 3.5 to 5.6 GPa does not correspond to a product in this range.

The 0.02 percent linear shrinkage figure is what allows a poured chock to hold alignment set before cure. Shrinkage at that level over a chock 100 mm deep is 0.02 mm, below the alignment tolerances the same assembly is verified to.

The crankcase as a pressure envelope under IACS UR M10

IACS UR M10 treats the crankcase enclosed by the frame box as a pressure envelope that must survive an explosion, and it sizes that duty against the relief valves fitted. UR M10.1 requires the crankcase, together with its doors, to withstand the explosion pressure with account taken of the relief valves required by UR M9, and UR M10.4 requires those valves to comply with UR M9. The two requirements are read together; neither stands alone.

Clause 5.2 sets the limit on the other side of the pressure range. Where forced extraction of the crankcase atmosphere is provided, the vacuum in the crankcase is not to exceed 2.5 x 10-4 N/mm2, that is 2.5 mbar. Excessive extraction draws in air through the seals and works against the oxygen-lean condition that keeps a crankcase mist below its explosive limit.

Monitoring is required above a power or bore threshold, and what happens at the alarm depends on engine speed. UR M10 clauses 8 and 9 require oil mist detection, or bearing temperature monitors or equivalent devices, on engines of 2,250 kW and above or with cylinder bore exceeding 300 mm. On low-speed engines the arrangement gives alarm and slow down. On medium- and high-speed engines it gives alarm and automatic shutoff. UR M10 fixes the speed bands used for that split: low speed below 300 rpm, medium speed 300 to below 1,400 rpm, and high speed 1,400 rpm and above.

Every large two-stroke in service falls in the low-speed band and above both thresholds, so the practical result on a main engine is alarm and slow down, with the detection equipment type approved and tested to UR M67.

Redundancy is per engine, not per ship. UR M10 clauses 12 and 13 require each engine to have its own independent oil mist detection arrangement and a dedicated alarm, readable from a safe location away from the engine.

Dual-fuel crankcase ventilation in UR M10 Rev.5

UR M10 Rev.5, dated November 2024, applies to engines certified or contracted on or after 1 January 2026, and its substantive change is the crankcase ventilation regime for dual-fuel and low-flashpoint-fuel engines. Clauses 5 and 6 carry that regime and cross-refer to UR M59.3.2.

The revision defines the lower explosive limit by reference to an external standard rather than leaving it to the engine builder: IEC 60079-10-1 (February 2021), paragraph 3.6.12. Fixing the LEL definition matters because a dual-fuel crankcase can hold a fuel gas as well as an oil mist, and the two have different explosive limits and different detection requirements.

The contracting date is what decides applicability, so a dual-fuel newbuilding contracted in 2025 and delivered in 2027 is assessed against the previous revision unless the builder elects otherwise.

Crankcase explosion relief valves under IACS UR M9

IACS UR M9 Rev.3, dated January 2005 with Corrigendum 1 of the same year, applies to engines of cylinder bore 200 mm and above, or crankcase gross volume 0.6 cubic metres and above. Both are alternative triggers, so a large-volume crankcase on a small-bore engine is caught by the volume limb.

Valve distribution follows bore, on the reasoning that a larger bore concentrates more crankcase volume per throw.

Source: IACS UR M9 Rev.3 (January 2005) with Corr.1.

Free area is specified twice over, per valve and in total. UR M9.2 sets the free area of each valve at not less than 45 square centimetres. UR M9.3 sets the combined free area of all valves at not less than 115 square centimetres per cubic metre of crankcase gross volume. A valve count that satisfies the distribution rule can still fail the combined-area rule on a large crankcase, so both are checked.

Opening behaviour is what separates a relief valve from a burst disc. UR M9.6 requires the valves to open quickly and be fully open at a pressure not greater than 0.02 N/mm2, that is 0.2 bar. UR M9.7 requires a flame arrester, so the vented products do not carry flame into the engine room. UR M9.8 requires the valve to be type tested to UR M66.

Builder practice sits above the UR minimum on large-bore engines. Everllence states that on the exhaust side of the frame box, a relief valve is mounted for each cylinder, and the WinGD X92-B operation manual describes one relief valve per cylinder on the exhaust side. Both arrangements satisfy the one-per-crankthrow rule for bores above 300 mm and put the discharge on a consistent side of the engine, which is what allows the venting arrangement to be routed as a single run.

Oil mist detection: the numbers in IACS UR M67

IACS UR M67 Rev.2, dated February 2015, sets the detection range and the alarm point for crankcase oil mist equipment, and both are stated in mg/l. Clause 6.3 requires a detection range of 0 to 10 percent of the lower explosive limit, and states that the LEL corresponds to an oil mist concentration of approximately 50 mg/l, about 4.1 percent oil by weight in the air mixture. Clause 6.4 requires the alarm set point to provide an alarm at a maximum level corresponding to not more than 5 percent of the LEL, or approximately 2.5 mg/l.

The unit is the trap. Milligrams per litre and milligrams per cubic metre differ by a factor of 1,000, so the UR M67 alarm point of 2.5 mg/l is 2,500 mg/m3. A secondary source that reports the alarm point as tens of milligrams per cubic metre has both the percentage and the unit wrong, and the resulting figure is out by more than an order of magnitude.

Read alongside UR M10, the requirement is a two-part one. UR M10 says which engines need detection and what the detection must do to the engine. UR M67 says what the equipment itself must be capable of, and the equipment fitted must be type approved and tested against it.

The alarm point sits at one twentieth of the explosive limit, so an oil mist alarm is not a report that the crankcase is about to explode. It is a report that a local hot spot is generating mist at a rate the crankcase ventilation is not clearing, and it is actionable long before the atmosphere is ignitable.

Documented crankcase explosion mechanisms

MAN Service Letter SL2022-730/PRP, “Prevention of crankcase explosions”, dated September 2022, cites UR M9, UR M10 and UR M66 in its rules of reference and records two casualty mechanisms that turn a crankcase explosion into an engine room casualty.

In the first, an old-type relief valve whose wire mesh did not act as a flame arrester allowed flame propagation into the engine room, and the result was a full engine room fire. In the second, the relief valve did not reclose after the first explosion, admitting fresh air and causing a second explosion that blew the crankcase doors off the engine. Valves type approved to UR M66 are designed to withstand two explosions, which is the design answer to the second mechanism.

The same service letter names the four relief valve makes type approved to UR M66 between 2006 and 2020: Hoerbiger Ventilwerke EVX, Mt. Halla Control Valves CR52, Ziincheol (former Hyunwoo SMT) HWG, and Prosave ERV (former Unitech).

One initiating mechanism in the letter sits upstream of the crankcase entirely. The main system lube oil filter may burst if it is not properly maintained, releasing retained particles into the main bearings and causing hot running and bearing collapse. A crankcase explosion investigation that stops at the crankcase misses that path.

Two operational rules in the letter are absolute. Venting pipes are to be in steel and at least 20 metres long. A flame arrester is never to be covered with plastic, paper or paint, a precaution aimed at yard and drydock practice rather than at running maintenance.

MAN has required bearing wear monitoring on its designed engines of bore 45 and above since 2008, with retrofit guidance in Service Letter SL2013-569/HWC. The 2022 letter also records that disabling the oil mist detector and ignoring its alarms has previously caused casualties, which is the reason UR M10 puts the alarm at a station away from the engine.

Fatigue cycles the frame accumulates

A two-stroke fires once per revolution per cylinder, so the frame sees one firing cycle per revolution and cycles per hour equal the shaft speed in rpm multiplied by 60. That single relation fixes the conversion between running hours and cycle count, and it is the check that catches an inconsistent fatigue claim.

Derived by Shipping-Wiki.com from the firing frequency of a two-stroke cycle. The speeds bracket the range published for current large-bore two-strokes.

The arithmetic is worth carrying because published statements about frame fatigue life are often quoted in hours without the speed they assume. An hours figure that implies a speed outside the engine’s own operating band is not describing that engine.

Cycle count alone does not settle a fatigue assessment. The stress range at the detail, the weld class of the joint, the S-N curve the classification society accepts for it, and the load spectrum the ship actually runs all enter, and they come from the engine builder’s analysis of the specific model rather than from a generic conversion.

Engine geometry of current large-bore two-strokes

Published bore, stroke and rated output figures anchor the structural discussion, because frame box height follows stroke and guide reaction follows both piston force and rod geometry.

Sources: MAN Marine Engine Programme and MUN2018-04-25 for the G95ME-C; the G60ME-C9.5 project guide; WinGD Marine Installation Manual X92DF, Issue 2021-09, Table 1-1.

The G95ME-C stroke is 3,460 mm. A figure of 3,260 mm appears elsewhere in the same engine programme for a different G-type and is not the G95 value.

MAN Energy Solutions now publishes as Everllence, and the 2025 and 2026 project guides carry that imprint with the engines branded Everllence B&W. Documentation issued before the change, including the MAN B&W project guides and the service letters cited here, retains the earlier name and remains current in substance.

Assembly and alignment sequence at newbuilding

Engine assembly proceeds from the bedplate upward, with the crankshaft installed and verified before the column and cylinder block go on, and with final chocking deferred until the ship is afloat. WinGD sets out the sequence in section 3.6.1 of the Marine Installation Manual for the X92DF.

1. The bedplate is lowered onto blocks between the chocking points and set slightly higher than its final position.
2. Preliminary alignment is made with wedges or jacking screws, with the coupling flanges parallel in the horizontal plane to a maximum deviation of 0.05 mm, and the engine coupling flange set 0.4 to 0.6 mm higher than the calculated position.
3. Bearing caps are placed, the turning gear is installed, and crankshaft deflections are taken and matched against the works Engine Assembly Records.
4. Bedplate level is checked longitudinally and diagonally with a taut-wire measuring device supplied by the engine builder, and the readings are compared with the works records.
5. The columns, cylinder blocks, running gear and scavenge air receiver are mounted.
6. The bearing caps are confirmed loose before the tie rods are tensioned.
7. Crankshaft deflection checks are repeated through the assembly.
8. Once assembly is complete, final alignment and chocking is carried out with the vessel afloat.
9. Final dimensions are witnessed by the engine builder’s and the classification society’s representatives and recorded on log sheets.

Step 6 is the one that is easy to get wrong and expensive to discover late. Tensioning the tie rods against clamped bearing caps locks a distortion into the main bearing line that the deflection check in step 7 will then report as a crankshaft fault.

Slipway installation adds three items to the sequence. The tie rods are centred and set exactly perpendicular to the bedplate before tightening; temporary side, fore and aft arresters are fitted; and upper-platform stays are rigged for the launch. All three exist because the hull moves during launch in ways it never moves again.

The deferral of final chocking to the afloat condition is the sequence’s governing constraint. A hull on blocks carries a different deflection pattern from a hull in the water, so a chock poured on the slipway sets the engine to a geometry the ship will not hold in service.

The four IACS requirements that govern the crankcase, read as one regime

Four Unified Requirements govern the crankcase enclosed by the frame box, and each answers a different question. Read separately they look like overlapping paperwork. Read together they form a single chain from the engine that needs protection, through the device that protects it, to the test that qualifies the device.

Chain compiled by Shipping-Wiki.com from the cross-references inside the requirements themselves: UR M10.4 points to UR M9, UR M9.8 points to UR M66, and UR M10 requires the oil mist equipment to be type approved and tested to UR M67. The regime as a whole, across engine types rather than for the frame box alone, is the subject of crankcase explosion protection .

The chain is what makes a single non-conformity consequential. A relief valve of the correct count and free area that is not type approved to UR M66 fails UR M9.8, and through UR M10.4 it leaves the crankcase enclosure without the basis on which its explosion-pressure duty under UR M10.1 was assessed. That is the kind of finding a machinery surveyor raises on a continuous survey of machinery cycle.

Where the frame box sits in the engine’s structural system

The frame box is one element in a load chain that runs from the combustion chamber to the ship’s double bottom, and each neighbouring element is treated in its own article.

Below it, the bedplate carries the crankshaft and the seating interface, and the flatness it presents is what the frame box is assembled against. The main bearing line inside it is covered with the crankshaft and main bearings , including the fatigue calculation route the classification societies accept.

The running gear the frame box guides is the defining feature of the configuration, and the crosshead engine architecture article covers why the crosshead exists and how the piston rod assembly is arranged around it. The seal between the crankcase below and the scavenge space above is treated with the piston rod stuffing box , which is what keeps cylinder oil and combustion products out of the crankcase atmosphere the oil mist detector reads.

Above the frame box, the cylinder frame houses the components covered in cylinder liner design , and the gas exchange those liners serve is covered in uniflow scavenging .

Two articles deal with what the surrounding ship does to the assembly. Engine alignment and bedplate flexure covers the hull deflection that changes bearing reactions under cargo and sea loading, which is the reason final chocking waits for the afloat condition. Torsional vibration analysis covers the rotating-system counterpart to the H-type and X-type lateral modes described above, and the two assessments are made separately because they load different parts of the structure.

Limitations of this article

This article describes the frame box and column of current Everllence, MAN B&W and WinGD large two-stroke crosshead engines. Several boundaries apply.

Engine-specific values are not published in the documents cited here. Tie rod and stay bolt diameter, free length, pre-tension value, hydraulic tensioning pressure and the tensioning sequence are given in the engine builder’s maintenance manual and the tensioning tool data sheet for the specific model. Peak cylinder pressure for a given rating is generated by the builder’s own calculation tools rather than tabulated in the project guide. None of those values should be taken from a generic source.

Medium-speed four-stroke engines are outside the scope. A trunk piston engine has no crosshead, no crosshead guide, and no three-tier frame box. The crankcase requirements in IACS UR M9, M10 and M67 do apply to those engines, and the speed bands in UR M10 are written for them, but the structural discussion here is not transferable.

Older cast-column engines differ in detail. Sulzer RD and RND series and early B&W engines were built with different column construction and different joint arrangements from the welded frame boxes described here, and work on them belongs to the original manufacturer’s service documentation.

Class rule detail varies by society. The IACS Unified Requirements set the floor that member societies implement, and each society’s own rules carry the survey scope, the inspection intervals and the acceptance criteria that apply to a particular ship. Confirm the requirement against the society and rule edition that classes the ship.

Fatigue and vibration assessment needs model-specific data. The cycle-count table here converts running hours to firing cycles and nothing more. A frame or stay bolt fatigue assessment requires the stress range at the detail, the applicable S-N curve, and the duty spectrum, all from the engine builder’s analysis.

Related Articles

• Engine Bedplate Construction
• Crosshead Diesel Engine Architecture Overview
• Marine Engine Crankshaft and Main Bearings
• Cylinder Liner Design for Two-Stroke Engines
• Piston Rod Stuffing Box Function
• Engine Alignment and Bedplate Flexure
• Engine Torsional Vibration Analysis
• Uniflow Scavenging in Two-Stroke Marine Engines
• Crankcase Explosion Protection on Marine Engines
• Engine Chocking and Holding-Down Arrangements