Dana Limited Slip Differential

Fig. 1 Two pinion Limited-Slip Differential - Model 44-IFS and Model 60-1U Trac-Loc:

FIG. 2 Four Pinion Limited-Slip Differential-Model 702U and 7OHD Powerlok. Design with Radial Grooves on Plates, Outer Dished Plate, and One Dished Disc per Clutch Pack (See Fig. 16 for Newer Design):

FIG. 3 Limited-Slip Differential Power Flow with Both Wheels Driving:

The limited-slip Dana Trac-Lok Model 44 IFS, Model 44 IFSHD and Model 60-1U two pinion axle (Fig. 1) and the four pinion Dana Power-Lok Model 70 (Figs. 2 and 4) have a power flow identical to the conventional rear axle, plus a more direct power flow which automatically takes effect as driving conditions demand. This more direct power flow is from the differential case to each axle shaft through clutches (Fig. 3).

A conventional differential transmits all of the ring gear torque through the differential gears to the axle shafts. Torque is at all times equal on the axle shafts, and if one wheel slips, the other wheel can only put out as much torque as the slipping wheel. The limited-slip differential is similar, except that part of the torque from the ring gear is transmitted through clutch packs between the side gears and differential case. The multiple-disc clutches, with radial grooves on the plates and concentric grooves on the discs, are engaged by a preload from dished springs and separating forces from the side gears. This occurs as torque is applied through the ring gear.

CAUTION: Model 60 clutch packs are phasing in discs with a special surface coating in place of the concentric grooves. Model 70 clutch packs are phasing in plates with the special surface coating In place of the radial grooves. Discs and plates of this design should be soaked in Additive Friction Modifier C8AZ-19B546-A (EST-M2C118-A) or equivalent, prior to assembly for twenty minutes.

Each clutch plate and disc pack consists of steel plates, set between the case and the side gear ring. The clutch plates, having external lugs, are locked to the differential case by the external lugs. The clutch plate that is installed next to the case is a Bellville spring plate. This plate is installed with the concave (dished) side against the case. The remaining clutch discs have internal spline teeth which lock to the splined hub on the side gear ring. Each clutch pack is premeasured for proper stack height. Do not separate packs and intermix the plates and discs from the packs. The side gear ring is, in turn, splined to the axle shaft and acts as a pressure plate against the clutch pack. Since the side gear ring fits against the pinions on the cross shaft (mate shaft), any outward force exerted by the mate shaft and its pinions will press the ring against the clutch pack and thus connect the differential case directly to the axle shaft.

Unlike the cross shafts of the conventional unit, the 4 pinion limited-slip differential mate shafts are not rigidly attached to the differential case, nor are they attached to each other. At both ends of the mate shafts there are two flat surfaces so arranged that they form a V which mates with corresponding V-shaped surfaces (ramps) cut in the shaft openings of the differential case. The mate shafts are assembled in the case with enough clearance so that when the case tries to rotate, they resist rotation. They are then forced to bear against one side of their V ramps. Since the two V ramps of one mate shaft point in a direction opposite to those of the other pin, the two shafts with their pinions will be forced apart as they resist rotation. This mate shaft movement compresses the clutch pack through the pinion gears and side gear rings.

When the differential case rotates in the opposite direction, the mate shafts will be forced to bear against the opposite side of their V ramps and will again be forced apart to apply against the clutch packs. Therefore, since the ramps are V-shaped, the clutches will apply during either forward or reverse operation. Likewise, the clutches will apply whether the power flow is from the differential case to the axle shafts, or from the axle shaft to the differential case.

FIG. 4 Limited-Slip Differential-Model 702U and 7OHD Power-Lok:

The amount of compression on the clutch plates will be proportionate to the load applied to the differential case and the resistance to turning offered by each mate shaft. For example, if the vehicle is driven straight ahead and the traction or load on both wheels is equal, both mate shafts will give equal resistance to the rotating differential case and will thus bear against their respective ramps with equal force. This equal movement of the mate shafts will cause them to exert equal pressure against both right and left clutch packs. Both axle shafts will, therefore, be locked directly to the case with equal force. Fig. 4 shows a cutaway view of a Model 70 axle Power-Lok differential assembly.

The limited-slip differential prevents momentary spinning of one of the wheels when it leaves the road because of a bump, or encounters poor traction because of a slippery road. Under these conditions, even though the traction load is relieved on the one wheel, the acceleration load is simultaneously applied to the differential case as the engine tries to spin the wheel.

When the rear axle is in a turn, the appropriate clutch releases automatically to allow normal differential operation as required. In the straight-ahead position, the differential case is driving both wheels and thus applies an equal load against both mate shafts. Since both mate shafts offer resistance to this load, both clutches are applied. On a turn, however, the outside wheel turns faster than the inside wheel. The outside wheel, instead of being driven by the case, now tends to drive the case. With the power thus relieved, the differential case releases its load against the outside wheel mate shaft which, in turn, releases its pressure against the outside wheel clutch pack. With the clutch released, normal differential action will take effect.

For a complete understanding of limited-slip operation, it is important to recognize two things:

1. If with equal traction, both wheels slip, the axle has done all it can do.

2. In extreme cases of differences of traction, the wheel with the least traction may spin after the axle has transferred as much torque as possible to the non-slipping wheel.