In practice the link pin joints in the centre of the toggle train, followed by the crosshead guides. The link pins carry the amplified clamp force through a rotation of only a few degrees, which is the amplitude at which grease is worked out of the contact rather than drawn into it. The crosshead guides carry the crosshead weight and the reaction from the toggle train, and they slide rather than rotate, so any loss of lubricant film shows up there quickly. Tie bar bushings on the moving platen usually wear more slowly, because the load is more evenly distributed and the alignment is better controlled. On a fast-cycling machine, expect the link pins and the crosshead guides to set the maintenance interval.

Running-in is a small, controlled amount of early wear that converts the initial line or point contact between a new bushing and its pin into a conforming area contact. It happens whether or not it is planned. Planned, it is carried out at reduced load and speed with adequate lubrication, and the bronze gives up the material rather than the pin. Unplanned, it happens at full clamp force on the first production shift, the contact pressure at the line of contact is greater than what the surface can carry, and the material that moves comes from whichever surface is softer or rougher. A tie bar is expensive, difficult to remove, and its surface condition governs the life of every bushing that runs on it.

For a bronze bushing to give its published service life, yes. The published counterface requirement for graphite-plugged bronze is 40–45 HRC or harder with a surface finish of Ra 0.8 µm or finer, and the same specification is the sensible default for a grease-lubricated bronze bushing on a toggle pin or a tie bar. Below that hardness, asperity contact during running-in and under shock load damages the pin rather than the bushing, and once a tie bar is scored it abrades every replacement bushing fitted to it. Hardening the pin or the bar costs a fraction of replacing a tie bar, and it belongs on the drawing rather than left to the shop floor.

Start from the projected bearing pressure on the pin rather than from the clamp tonnage alone, because the toggle geometry amplifies the cylinder force by a factor that changes through the stroke. Where pressure is high and the section is compact, CuZn25Al5 (CC762S comparable, comparable to C86300) at 780 MPa typical and 210 HB typical is the usual starting point. Where the geometry allows more bearing area and sliding behaviour governs, CuSn12Ni2-C (CC484K, comparable to C91700) at 300 MPa minimum tensile and 180 MPa minimum yield gives good conformity and galling resistance. Confirm every figure against the material test certificate, and note that published minimum values are specification floors rather than expected production values.

It can be, and it is the input most often missing from the specification. The clamp unit sits close to a mould running at elevated temperature, and heat conducts into the platen and along the tie bars. The number that matters is the metal temperature at the bushing, not the mould temperature setting. Where the measured metal temperature at the bushing stays low, a leaded bearing bronze is appropriate; where it rises, the alloy choice has to change, because published maximum mechanical service temperatures for leaded tin bronze classes sit far below the melting point of lead at 327 °C. Measure the metal temperature at the bushing bore during steady production before committing to an alloy.

Only if the existing pins already meet the counterface requirement. A graphite-plugged bronze bushing is specified against a counterface of 40–45 HRC or harder at Ra 0.8 µm or finer, and against a dry-running allowable surface pressure of approximately 100 N/mm² with a sliding speed ceiling of 0.5 m/s. If the pins are soft, scored or rough, fitting a plugged bushing will not restore the surface — it will run against the damaged one. Measure hardness, diameter, roundness and surface finish first. Where the pins are sound, the change is straightforward; where they are not, the pin work is the job and the bushing is the smaller part of it.

Through clamp repeatability. Every cycle the clamp closes, builds tonnage and holds it while the cavity is filled and packed. If friction in the toggle joints varies from cycle to cycle — because the joint is in a stick-slip regime, or because the lubricant film breaks down intermittently — then the point at which the toggle reaches full extension, and therefore the tonnage actually applied, varies with it. Applied tonnage that drifts below what the cavity needs shows as flash at the parting line; tonnage that varies between shots shows as dimensional variation in the part. A bushing with a stable coefficient of friction is therefore a process-capability item rather than a maintenance item.