CuSn8P and CuSn6 Wrapped Bushes on Excavator CrawlersAlloy choice, lubrication pocket codes, and the speed ceiling that decides where a thin-wall sleeve belongs

CuSn8P = EN CW459K, UNS C52100 · CuSn6 = EN CW452K, UNS C51900 · wrapped thin-wall product standard DIN 1494 / ISO 3547-1 · calculations run against a PV reference of 2.8 N/mm²·m/s for grease-lubricated wrapped bronze, with a dynamic pressure reference of 40 N/mm²

Two phosphor bronzes carry most of the wrapped bushing volume on a crawler undercarriage, and the gap between them in strip form is narrower than the alloy names suggest. What decides the choice is the duty mix at the joint, not the tin content. Three questions settle most positions: how the joint moves, what reaches the sealing surface, and whether grease can be replenished. This article works through those three, then sets out the speed ceiling that removes a wrapped sleeve from a position before any material question is asked.

1  Scope and reading instructions

2  How far apart the two alloys actually are

Association data for rolled strip, converted from ksi at 6.895 MPa per ksi. These are typical values for the temper, not guaranteed minima.

Source class: industry association rolled-strip typical values, UNS C52100 and UNS C51900. Temper designations follow the US strip convention.

A single-digit difference across the whole temper series changes how the alloy question should be asked. A figure in the region of 5 to 8 % does not by itself decide whether a joint survives, so the selection argument has to be made on the duty mix. Three observations follow from the table and from published composition limits.

• Shock-dominated pivots. Boom and arm joints, and rock-reacting points, see load reversals rather than steady load. Fatigue behaviour and yield margin decide the outcome ahead of the last few percent of tensile strength, and CuSn8P is the normal starting point there.
• Grit-dominated slow oscillation. CuSn6 is reported roughly 10 to 20 HB softer than CuSn8P in the same condition. A softer, more compliant surface accommodates embedded particles without scoring the pin. Where contamination reaches the sealing surface and the motion is slow, that property can outweigh the strength difference.
• Cost-sensitive, lightly loaded positions. Where the load is well inside the allowable range, the alloy choice can follow availability and price rather than performance. Both alloys carry the same catalog working figures for this product family, so the catalog tables do not separate them.

3  Designations: what to write, and what the codes do not mean

Most material errors on wrapped sleeve orders are naming errors, and three of them account for nearly all of them.

Rows 1 to 3: designation systems EN and ASTM, and the industry association composition data behind them. Row 4: association published minimums for cast UNS C90300 against association typical values for wrought strip. Row 5: product-family coding within the DIN 1494 and ISO 3547 wrapped thin-wall range.

3.1  What the pocket codes actually control

Pockets hold a larger grease volume and build a film faster at start-up, which extends the interval between replenishment. That suits shock-loaded positions and positions where a grease line is not convenient. Through holes suit positions with controlled injection after assembly and an end-face seal. A separate solid-lubricant variant, graphite-plugged, belongs to a different system and is not interchangeable with the oil-lubricated pocket structures.

Product-family coding follows the DIN 1494 and ISO 3547 wrapped thin-wall range. Structure dimensions for lubrication holes and pockets are covered in ISO 3547-3; material designations are covered in ISO 3547-4. Other manufacturers use different letters for the same constructions, which is a further reason to compare on the three elements above rather than on a letter sequence.

4  Form follows access: plain, flanged, split

The alloy decides how the sleeve resists the load. The form decides whether it can be fitted at all, and whether it can be replaced on site.

Construction descriptions follow the wrapped thin-wall plain bearing range and general component practice for this equipment class. Wall, flange and clearance dimensions are set by the specific form and are confirmed against the part drawing rather than by a catalog figure.

5  The speed ceiling

Specific pressure and PV answer different questions. Pressure governs a pivoting joint; PV governs continuous rotation. For a rotating sleeve the ceiling follows from PV in two steps: the allowable sliding speed is the PV reference divided by the specific pressure, and the rotational speed follows from that sliding speed and the bore.

At 40 MPa the sleeve sits at the dynamic pressure reference, and the resulting ceiling is about 5.6 rpm. The table below extends the same calculation across bore sizes.

Method: v = 2.8 / p with p in MPa; n = 60·v / (π·d) with d in metres. Values in rpm, rounded. The PV reference is a catalog figure for grease-lubricated wrapped bronze, not a certified allowable; oil-lubricated and thick-wall forms permit different figures and are calculated separately.

The trend across the table is the practical result: the larger the bore, the lower the ceiling. A large wrapped sleeve is therefore an oscillating pivot component. If a large-bore position is specified for continuous rotation above roughly ten rpm, the answer is a change of form — a solid thick-wall cast sleeve, a steel-backed bimetal liner or a rolling element — rather than a change of alloy.

6  The sliding speed inside a roller bore

Track speed and the sliding speed at a roller bore are different quantities, and conflating them overstates the duty. The lower run of the track is stationary on the ground and the roller tread rolls without slip, so the tread speed equals machine travel speed. The bush is pressed into the roller body and rotates with it against a fixed shaft, so the sliding speed at the bore is the roller surface speed multiplied by the shaft-to-roller diameter ratio.

Bore sliding speed = machine travel speed × (shaft diameter / roller diameter). A ratio of 0.15 to 0.25 is the range reported for crawler roller proportions. Values in m/s.

A representative 20-tonne-class check closes the argument. With roughly 200 kN on seven or eight bottom rollers, each roller sees about 25 to 30 kN. At a bore of 80 mm and a length of 100 mm that is a specific pressure near 3 to 4 MPa; at 5 km/h and a ratio of 0.20 the bore sliding speed is about 0.28 m/s, giving PV of about 1.0 against the 2.8 reference. The speed term has margin. What limits life at this position is shock, contamination and seal performance, so the selection effort belongs on embeddability and sealing rather than on alloy strength.

Worked example at the stated assumptions. Not measurements from a particular machine.

7  Positions where a bronze sleeve is not the answer

Three positions are routinely quoted as bronze bushing positions and are not, on the usual constructions.

Hardness ranges as published by component suppliers for these constructions. They describe common practice for this equipment class and do not describe the layout of any particular machine.

8  Selection sequence

1. Fix the motion type. Continuous rotation, oscillation or reciprocating slide. Screen the position against Table 4 immediately: continuous rotation above the ceiling means changing the form, not the alloy.
2. Fix what reaches the surface. Grit, mud slurry, seawater or acidic water. Grit and a slow stroke favour the more embeddable alloy; seawater or acidic water moves the choice to aluminium bronze.
3. Fix lubrication access. A reachable grease fitting supports a pocket or through-hole structure; an inaccessible position supports a solid-lubricant grade.
4. Check the mating pin. Hardness, roughness and surface condition are measured, not assumed. A scored pin is repaired before a new sleeve is fitted, since a graphite or bimetal running surface places higher demands on the counterface.
5. Fix the form. Axial location required or not; shaft withdrawable or not. This decides plain, flanged or split.
6. Write the order in full. Standard, designation, product form, temper, bore, length, flange dimensions, lubrication structure and surface requirement on the mating pin.

8.1  Information required before a quotation

• Machine class and positionThe same position name can refer to different parts across manufacturers
• Original constructionBronze sleeve, bimetal, hardened steel sleeve or rolling element — this sets the replacement material family
• DimensionsBore, outer diameter, length, flange size, thrust face
• Pin diameter, hardness and roughness, measuredPublished hardness figures differ between sources; see Section 2
• Motion descriptionContinuous rotation, swing angle with cycles per minute, or reciprocating stroke
• Load basisMachine weight and roller count, or circuit pressure
• Lubrication and sealingGrease fitting, sealed chamber or no lubrication; seal type and its condition
• Contamination and shockSand, rock dust, slurry, seawater; breaking or drilling duty
• Governing material standardEN or ASTM — the phosphorus window differs; see Table 2 item 3

9  Service limits worth recording

• Clearance. Published replacement thresholds differ by construction; figures of 0.05 to 0.12 mm for idler bushes and 0.1 to 0.3 mm for bimetal liners circulate in component literature. Use the figure attached to the machine documentation and replace the sleeve and pin as a pair.
• Seals. Floating seals and lip seals are replaced once removed. A sealing face that has been disassembled starts to leak within weeks.
• Temperature. On rollers, a bearing temperature above roughly 20 °C relative to ambient during continuous travel calls for stopping, cooling down and investigating lubrication and sealing.
• Chain inspection. Inspect pin and bushing at intervals of roughly 250 to 500 hours, and treat pitch growth as the primary wear measure.
• Worn bores. Where a housing bore is out of tolerance, an oversize sleeve with a matching oversize pin restores the original motion. Re-bore the housing and do not reduce the pin diameter instead.

Service thresholds as published in component and aftermarket literature for this equipment class. Values differ between sources and machine types; verify against the maintenance documentation for the machine.

10  Frequently asked questions

• Is CuSn6.5 a standard copper alloy grade?No. CuSn6.5 does not appear in the EN, UNS, JIS or GB designation systems. The nearest standard grade is CuSn6, EN CW452K, UNS C51900, which corresponds closely to the Chinese grade QSn6.5-0.1. The usage in supplier documents most likely follows the Chinese naming habit. Write the full designation and the governing standard on the drawing and on the purchase order.
• Do FB090 and FB092 identify the alloy?No. They identify the lubrication structure of the wrapped sleeve. FB090 describes a diamond pattern of oil pockets, FB092 describes through holes for injected grease, and the same pattern is available in either alloy. Compare parts on three elements: bore and length, alloy designation, and lubrication structure.
• How much stronger is CuSn8P than CuSn6 in wrapped strip?In the same temper the difference is modest. Association data gives about 531 MPa for UNS C52100 against 496 MPa for UNS C51900 in the half-hard temper, a difference of 6.5 %, narrowing to 4.5 % in the spring temper. The alloy choice therefore rests on the duty mix, mainly the balance between shock loading and grit ingress, rather than on tensile strength alone.
• What is the allowable rotational speed of a wrapped bushing?It follows from PV. With a grease-lubricated wrapped bronze sleeve, a commonly quoted catalog ceiling is 2.8 N/mm²·m/s. Specific pressure equals radial load divided by bore times length, so the allowable sliding speed is 2.8 divided by that pressure, and the rotational speed follows from 60 multiplied by that speed divided by pi times the bore in metres. A 120 mm bore at 20 MPa works out at about 22 rpm, and a 240 mm bore at 40 MPa at about 5.6 rpm. Larger bores suit oscillating pivots rather than continuous rotation.
• Is a flanged wrapped bushing a thrust bearing?No. Flange thickness is approximately the sleeve wall thickness, so axial capacity is governed by flange bending rather than by the compressive strength of the alloy. On idler and tensioner positions the flange locates the part axially and carries light thrust. Where significant axial load is present, provide a separate thrust face such as a shoulder, a thrust washer or a rolling element.
• When is a split bushing used instead of a solid sleeve?Where the shaft cannot be withdrawn, so the sleeve has to be fitted around the assembled joint — typically idler bores, oscillating shafts and hydraulic breaker pivots. A split introduces reduced stiffness and edge loading at the cut. Keep the split line out of the loaded zone and use a stepped or dowelled profile to stop the two halves rotating relative to each other. A split sleeve is not suited to high-speed continuous rotation.
• Can a wrapped bronze sleeve be used in a track chain pin joint?Not as a thin-wall wrapped sleeve. That joint is normally a case-hardened or induction-hardened steel pin running in a hardened steel bushing, with wear surfaces reported in the 55 to 62 HRC range and a sealed grease chamber or a dry arrangement. Wear appears as pitch growth, and the first cause is seal failure and loss of lubrication rather than the choice of material. Specify the chain link on the steel system and check it against the relevant chain standard.
• Is UNS C52100 enough to define the alloy on a purchase order?Not on its own. EN CW459K and ASTM C52100 share the UNS number but carry different phosphorus windows, 0.20 to 0.40 % against 0.03 to 0.35 %. State the governing standard on the order and on the material certificate, together with the product form and temper.