1. How Solid Lubricant Inserts Work

When two solid surfaces slide against each other, friction and wear originate from four mechanisms: surface adhesion or micro-welding, ploughing by asperities or hard third-body particles, surface deformation, and adhesive wear. Adhesive wear occurs when localised bonds form and break at the contact points; as sliding continues the material must shear along its weakest plane, and this repeated bonding-and-separation cycle removes material.

Solid lubricants interrupt that cycle. In a plugged bronze bushing, a running-in film together with PTFE or graphite inserts creates a physical separation between the sliding layer and the mating shaft. Because the bearing retains a lubricating film even while stationary, it starts at low friction and does not depend on an external oil film being present at the moment load is applied.

Solid lubricant plugs

PTFE- and graphite-based plugs are manufactured to high density for consistent lubricity, then embedded into pockets in the bronze support structure. Their macro-distribution pattern is laid out so that adjacent plug rows overlap in the direction of motion — rotating, oscillating or linear — which is what allows lubricant to be transferred continuously across the whole wear path rather than in isolated stripes. The plug materials are chemically stable and do not react electrolytically or chemically with the bronze body or the mating shaft, including in high-humidity and fully submerged service.

The running-in film

Bushings are supplied with a thin factory-applied film of solid lubricant, 15–20 µm thick, deposited after the plugs are inserted and the bore is finish-machined. This film should not be removed. If it is damaged during assembly it can be repaired on site with a compatible spray lubricant.

During initial movement part of the film transfers to the mating surface. This begins a transition phase that continues until solid lubricant is gradually released from the embedded pockets. The transfer process delivers three practical benefits:

• a consistently low coefficient of friction from the first cycle;
• minimal stick-slip behaviour under slow or reversing motion;
• immediate full load-bearing capacity, with no break-in under reduced load.

Why not simply use grease?

Self-lubricating bronze generally outperforms a conventionally lubricated bearing wherever the lubricant film is hard to maintain. Under high load, long dwell times, static contact, and even during ordinary rotation or oscillation, grease and oil can be squeezed out of the contact zone. Once that happens friction rises, wear accelerates, and the bearing fails early. Plugged bronze removes that failure mode entirely.

2. Standard Bronze Support Alloys

VIIPLUS builds plugged bushings on three standard cast bronze grades. Selection is normally a trade-off between load capacity, corrosion resistance and machinability.

Casting route and section size

A practical rule used across our plugged bronze range: bushings with a bore above 180 mm are produced from centrifugally cast bronze (ASTM B271 C86300), while bores below 180 mm are produced from continuously cast stock (ASTM B505 C86300). The two routes give different mechanical results for the same nominal grade, which is one reason published tables from different sources can disagree.

3. Tin Bronze Grades C90700 and C93800

Beyond the manganese and aluminium bronze bodies, two continuously cast tin bronzes are widely specified for bearing and wear parts. Both are covered by ASTM B505/B505M and are stocked as solid bar, tube and rectangular section.

^a Nickel value includes cobalt. For C90700 the copper minimum may be calculated as Cu + Ni. Cu + sum of named elements: 99.4 % min. for C90700, 99.0 % min. for C93800.

Choosing between them

C90700 is a high-tin bronze with roughly twice the tensile strength of C93800. It is the conventional choice for bearings under heavy load at relatively low speed, worm gears and worm wheels, gearboxes and speed reducers. Its machinability rating of 20 makes it comparatively slow to machine.

C93800 trades strength for embeddability. With 13–16 % lead dispersed through the copper-tin matrix it machines at rating 80, tolerates foreign matter by absorbing it rather than scoring the shaft, and keeps running when the lubrication boundary is lost. It is the standard answer for moderate-pressure general service bearings, pumps handling abrasive or acidic fluids, and large marine bearings. Note the low incipient melting point of 316 °C — it sets a hard ceiling on allowable operating temperature and on any thermal processing.

^b Brazing is performed within the hot-short range, so strain must be avoided during brazing and cooling.

4. High-Strength Nickel-Aluminium Bronze Reference Data

For the most heavily loaded bushings, nickel-aluminium bronze is the alternative to manganese bronze. The table below reproduces published property data for the continuously and centrifugally cast nickel-aluminium bronze grade identified by the SAE aerospace material specification AMS 4880 (corresponding UNS designation C95510).

5. Tribological Properties, Fits and Tolerances

The values below apply to the standard plugged bronze construction on a C86300 support body with PTFE inserts.

Recommended fits

For bearing bores above 200 mm, tolerances must be calculated by application engineering against the parameters of each application — the standard table above is not valid at that scale, and special fitting instructions may apply.

Mating shaft condition

Bearing life is strongly influenced by the hardness, roughness and surface treatment of the shaft. The following is a practical starting point for shaft specification.

6. Flanged Bush Dimensions — SOBF Series

The SOBF series covers metric flanged, self-lubricating bushes in 115 standard sizes, from 8 mm to 160 mm bore. The flange locates the bushing axially and takes thrust load, which makes this the preferred form where the assembly sees axial movement as well as radial load.

Scroll within the table for the full size range. Dimensions are nominal manufacturing references; confirm against the released drawing before ordering. Non-standard bores, lengths and flange diameters are produced to order.

7. Installation and Inspection

Press fitting into the housing

Solid bronze bushings are installed by pressing into a machined H7 housing bore. The following points determine whether the bore ends up round, concentric and at the specified clearance:

• machine an insertion bevel on the housing bore, and clean and deburr all parts before assembly;
• lubricate the outside diameter of the bushing before pressing;
• check axiality between the installation space and the centring axis of the bushing;
• where two bushings support one shaft, align them to each other;
• always press with a counter mandrel of suitable diameter so the force is taken on the bushing end face, never through the bore.

Installation is carried out on a hydraulic press or a mechanical arbor press. For wrapped and rolled steel-backed bushings, the maximum press resistance can be estimated from the strip thickness and bushing length: F = 300 N × L at 1 mm, 500 N × L at 1.5 mm, 700 N × L at 2 mm and 900 N × L at 2.5 mm, with L in mm. For bushing diameters below 55 mm, an installation ring of 0.3–0.4 mm is recommended to keep the bushing square in the bore.

Interference and retention

An interference fit serves two purposes: it prevents the bushing from rotating or moving axially in the housing, and it holds the outside diameter in full contact with the bore so that load is carried and frictional heat is conducted away. Aim for a contact area of more than 70 %.

Two methods are used where service pressure is high enough to risk the bushing creeping in the housing: increase the outside diameter and correct the resulting bore closure with a calibrating mandrel, or apply an anaerobic retaining compound to the housing bore to raise the bond strength. Chamfer both the bushing outside diameter and the housing bore so the bushing enters without shaving.

Cold assembly for heavy interference fits

1. Required: a coolant — liquid nitrogen or dry ice — and an insulated container of adequate size with insulation on all sides.
2. Standard freezing temperature is −40 °C to −70 °C.
3. Freezing time is normally one hour or more; extend it where the fit tolerance is tight. A short freeze combined with hydraulic pressing is also effective.
4. Re-measure the bushing outside diameter and the housing bore before assembly. An anomaly missed at this stage can cause serious failure in service.
5. Insert the bushing into the housing in one quick, rotating motion. If it stops part way the dimensions recover and withdrawal and re-assembly become very difficult.
6. Apply lubricant to the friction surface.

In winter, or wherever the ambient temperature difference is small, warming the housing to 20–30 °C is used instead.

Dimensional inspection of wrapped bushings

Rolled and wrapped bushings have a split seam and are therefore not truly round in the free state. Since the outside diameter is an interference fit that takes the shape of the housing, inside and outside diameters must not be measured in the free condition — dedicated gauges are required. The applicable method is set out in ISO 3547 (which replaced DIN 1494), Part 2:

• Test A — outside diameter. Two ring gauges, go and no-go. The bushing must pass the go gauge under a maximum hand force of 250 N and must not enter the no-go gauge under the same force. Accuracy suffers if the bushing is out of round or if closing the seam alone exceeds 250 N; in those cases use a combination of press-in force and wall thickness checks instead.
• Test C — inside diameter. Press the bushing into a reference ring gauge, then check the bore with a three-point instrument or go/no-go plug gauges. The go plug must pass at no more than 250 N hand force and the no-go plug must not pass. Note that pressing into the reference ring may permanently deform the outside diameter, so the bushing cannot then be used in production.
• Test D — outside diameter of large sizes. Measured with a precision measuring tape.
• The wall thickness method may be substituted for Test B, but the two must not be applied together.

8. Applications

Plugged bronze is specified wherever maintenance access is poor, the duty cycle includes long static periods, or the environment defeats conventional lubrication.

Custom bearing forms

Standard cylindrical bushes are only the starting point. VIIPLUS manufactures: cylindrical bushes, flanged bushes, thrust washers, sliding plates and tailored bearing designs to drawing, including special materials where the standard alloys are not suitable. For service above 250 °C, graphite inserts are offered in place of PTFE.

9. Selection Method

Sizing a plain bearing starts with the projected load area and the pU limit. The load area is the bore multiplied by the length (d × L), the specific load is p = F / (d × L), and the pU factor is the product of specific load and sliding speed:

p (N/mm²) × v (m/s) = pU (N/mm² · m/s)

Sliding speed is derived from the rotational speed, or from the oscillation angle and frequency for oscillating motion. As a first screening check for dry-running designs, verify that p stays within the dynamic limit, that v stays within the maximum sliding speed, and that the resulting pU stays within the maximum pU factor for the intended load profile — continuous, alternating or short-term. Service life in dry operation falls roughly in inverse proportion to the pU value, so a design run close to the limit will have a proportionally shorter life.

Correction factors are applied for load level, operating temperature, shaft diameter and mating-shaft material. Because so many variables interact, the result is an estimate: where the duty cycle is critical, validate the selection under service conditions before committing to production.