Load measurement tool

Abstract

A dummy / test unit 300 that mimics the shape and size of a transmission shaft and is inserted in its place to estimate (pre)load on a transmission shaft of an electric drive unit 10 of a vehicle. There

Description

TECHNICAL FIELD The present disclosure relates to a load measurement tool and to a method of estimating a load on a transmission shaft of an electric drive unit. BACKGROUND In an electrically propelled vehicle (either a battery electric vehicle propelled entirely by electric propulsion or a hybrid electric vehicle capable of being propelled by either or both of an electric motor and an internal combustion engine), an electric drive unit (EDU) is used to provide electric propulsion to the vehicle. The electric drive unit comprises an electric motor, powered by a vehicle battery, and a transmission including various gears and shafts. A transmission shaft is held in place within the EDU by a pair of taper roller bearings (one at each end of the shaft). It is important that each transmission shaft is supported in place within the EDU with a suitable amount of preload (the amount offeree acting between the shaft and the bearings). If the transmission shaft is held too loosely, this may cause issues with the transmission shaft deflecting under the application of load when the motor is operating, whereas if the transmission shaft is held too tightly, rolling resistance will increase and efficiency reduce. The preload on the shaft can be adjusted by a shimming process in which a suitable thickness of shim(s) is selected and placed behind one or both of the bearings. At present this is achieved by measurements made on the shaft with the EDU partially disassembled. This method is inaccurate, because the preload is in practice dictated by the compression of the shaft (via the bearings) between the assembled upper and lowers parts of the housing. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a load measurement tool and a method of estimating a load on a transmission shaft of an electric drive unit as claimed in the appended claims. According to an aspect of the present invention there is provided a load measurement tool, comprising: a first bearing support, for engagement with a first set of bearings; a second bearing support, for engagement with a second set of bearings; a sensor, mounted with respect to the first and second bearing supports and configured to output a signal indicative of a load exerted on the load measurement tool. In this way, a load (preload) exerted on the load measurement tool (which is indicative of the load which would be exerted on an equivalent shaft) can be measured while engaged between the first and second sets of bearings, representing a better approximation of the likely preload in actual use. The sensor may be a load cell, which in use is compressed between the first bearing support and the second bearing support, the signal output from the load cell being indicative of an axial force exerted on the load measurement tool via the first and second sets of bearings. The first set of bearings may be mounted between the first bearing support and a first part of an electric drive unit, and the second set of bearings may be mounted between the second bearing support and a second part of the electric drive unit. The first part of the electric drive unit may be a first casing part of the electric drive unit and the second part of the electric drive unit may be a second casing part of the electric drive unit, and the load measurement tool may be configured to be braced between the first casing part and the second casing part via the first set of bearings and the second set of bearings (when the first and second casing parts are joined). The first bearing support and the second bearing support may be coupled together, and the load cell may be mounted between a portion of the first bearing support and a portion of the second bearing support. The load measurement tool may comprise a housing disposed around at least a portion of the first bearing support and the second bearing support. The housing may comprise an external threading for engagement with a gear of a transmission. The load measurement tool may comprise a position sensor, for measuring a displacement from a fixed position with respect to the first bearing support to a fixed position with respect to the second bearing support. The position sensor may be a light sensor. The tool may have a shape and stiffness representative of a transmission shaft. In this way, the present technique is able to provide a reusable dummy shaft that can be assembled in place of the real part and is representative enough (in form and stiffness) to use real bearings and measure the preload with an installed load cell. The first set of bearings and / or the second set of bearings may be taper roller bearings. According to another aspect of the present invention, there is provided a method of estimating a load on a transmission shaft of an electric drive unit using a measurement tool according to the above, comprising the steps of: providing a second casing part of the electric drive unit; mounting the measurement tool to the second casing part via the second set of bearings; mounting a first casing part onto the second casing part, to trap the measurement tool between the first and second casing parts; and measuring a load exerted on the load measurement tool by the first and second casing parts via the first and second sets of bearings. The method may comprise a step of inserting a shim between one or both of the first casing part and the first set of bearings, and the second casing part and the second set of bearings. The method may comprise causing the measurement tool to rotate during the measurement step, or between subsequent measurement steps. To achieve this, the method may comprise winding one or more wires from an output of the load cell around the measurement tool in a first direction prior to the measurement step, wherein the step of causing the measurement tool to rotate comprises rotating the measurement tool in a second direction opposite of the first direction. Unlike the existing shimming process which dictates shim selection for the transmission taper bearings using height measurements with the unit disassembled, the proposed technique can be carried out on an assembled electric drive unit. In particular, the present technique provides a method for determining the achieved preload once the two main housing components (bracing the transmission shafts) close and are bolted together. This provides a feedback loop to denote the achieved preload of the transmission assembly with a given shim. This preload tool is designed to be equivalent to the transmission shafts (intermediate / lay shaft and differential) and can be fitted in place of them. The tool is able measure the preload exerted on the shafts via it’s load cell and output wiring (to be routed through the e-machine cavity). Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic illustration of a vehicle which comprises an electric drive unit having one or more transmission shafts tested using the present technique; Figure 2 shows a schematic illustration of part of an electric drive unit in which a transmission shaft is disposed; Figure 3 shows a schematic illustration of the same part of an electric drive unit, but with a preload measurement tool disposed in place of the transmission shaft; Figure 4 shows the preload measurement tool in more detail, in side view; Figure 5 shows a 3D view of the preload measurement tool of Figure 4; and Figure 6 is a schematic flow diagram of the overall method of measuring preload. DETAILED DESCRIPTION Referring to Figure 1, a vehicle 1 is shown which comprises two electric drive units 10, one at a front axle of the vehicle 1 and one at a rear axle of the vehicle 1. The electric drive units 10 are electrically connected to, and powered by, a vehicle battery 15. In alternative embodiments a single electric drive unit 10 may be provided, driving either or both of the front axle and the rear axle, or alternatively each wheel of the vehicle may be provided with its own electric drive unit 10. In the example shown, the vehicle 1 is a battery electric vehicle propelled electrically, and lacking an internal combustion engine. However, the vehicle 1 could alternatively be a hybrid electric vehicle comprising both one or more electric drive units 10 and an internal combustion engine. The or each electric drive unit 10 comprises an electric motor powered by the vehicle battery 15, and an output shaft (not shown) for conveying propulsion torque to a driveline (not shown) of the vehicle 1. Referring to Figure 2, a portion of an example electric drive unit 10 is shown. The electric drive unit 10 can be seen to comprise an upper casing 110, a lower casing 120, gears 130a, 130b, and a transmission shaft 200. In practice, more than one transmission shaft 200 may be provided. The transmission shaft 200 provides a mechanical linkage between the gear 130a and the gear 130b (the latter of which is mounted onto the transmission shaft 200). The transmission shaft 200 is rotatably mounted between the upper casing 110 and the lower casing 120 via (taper) roller bearings 140a (mounting an upper portion of the transmission shaft to the upper casing 110) and (taper) roller bearings 140b (mounting a lower portion of the transmission shaft 200 to the lower casing 120). It is important that the transmission shaft 200 be supported by the bearings 140a, 140b in a manner which is not too tight (over compressed) nor too loose (under compressed). That is, the distance between the bearings 140a at the upper casing 110 and the bearings 140b at the lower casing 120 should be carefully controlled to achieve the correct application of pressure onto the transmission shaft 200. This amount of pressure I force is referred to as “preload”. If the transmission shaft 200 is held too loosely, this may cause issues with the transmission shaft 200 deflecting under the application of load, whereas if the transmission shaft 200 is held too tightly, rolling resistance will increase and efficiency reduce. The distance between the bearings 140a, 140b can be controlled by placing shims behind one or both of the bearings 140a, 140b (that is, between the bearings 140a and the casing 110, or between the bearings 140b and the casing 120). This reduces the distance between the bearings 140a, 140b, thereby adjusting the preload. Existing shimming processes dictate shim selection for the transmission taper bearings, from height measurements with the unit disassembled (for example the upper casing 110 removed). In particular, a transmission shaft is mounted onto the bearings 140b, and weights added onto the shaft to approximate the force which would have been exerted by the bearings 140a and the upper casing 110. With this existing method there is no way to determine the achieved preload once the two main casing components 110, 120 4 (bracing the transmission shafts) close and are bolted together. As a result, there is no feedback loop to denote the achieved preload of the transmission assembly with a given shim. In contrast, the present technique utilises a reusable dummy shaft that can be assembled in place of the real part (with the casing fully closed) and is representative enough (in form and stiffness) to use real bearings and measure the preload with an installed load cell. Since the measurement tool (dummy shaft) is held between the two casing parts and the two sets of bearings, it can be held more concentrically than with the conventional techniques. Figure 3 illustrates such an arrangement. In particular, in Figure 3 a (pre)load measurement tool 300 is disposed within the electric drive unit 10, mounted between the bearings 140a, 140b. As such, the measurement tool is configured to be braced between the first casing part 110 and the second casing part 120 via the first set of bearings 140a and the second set of bearings 140b. As will be explained subsequently, the measurement tool 300 approximates the dimensions (shape and size) of the shaft 200 from Figure 2, and comprises two parts, axially movable with respect to each other and compressed towards each other by the first and second casing parts 110, 120 via the bearings 140a, 140b, and a sensor which measures this compression. Referring to Figures 4 and 5, respectively providing a cross-sectional view and an external view of the load measurement tool 300, the tool 300 can be seen to comprise a first bearing support 310, for engagement with the first set of bearings 140a, and a second bearing support 320, for engagement with the second set of bearings 140b. As a result, the first set of bearings 140a is mounted between the first bearing support 310 and a first part of an electric drive unit, and the second set of bearings 140b is mounted between the second bearing support 320 and a second part of the electric drive unit. More specifically, the first part of the electric drive unit is the first (upper) casing part of the electric drive unit and the second part of the electric drive unit is the second (lower) casing part of the electric drive unit. A sensor 360 is mounted with respect to the first and second bearing supports 310, 320, and is configured to output a signal indicative of a preload exerted on the preload measurement tool 300. The sensor 360 is a load cell, which in use is compressed between the first bearing support 310 and the second bearing support 320, and the signal output from the load cell is indicative of an axial force exerted on the load measurement tool 300 via the first and second sets of bearings 140a, 140b. In practice, wires (not shown) will be attached to the sensor 360, and will extend through and out of the electric drive unit to a measurement device which will read and output (display) the sensor signals. The first bearing support 310 and the second bearing support 320 are coupled together (rotationally, via a housing 330), but are movable (axially) with respect to each other. The load cell 360 is mounted (indirectly in this case) between a portion of the first bearing support 310 and a portion of the second bearing support 320. The second bearing support 320 comprises a first part, at one end thereof (the end distal from the first bearing support 310) having a first diameter, a second part, at the opposite end thereof (the end proximate to the first bearing support 310) having a second diameter, and a third part, between the first part and the second part, having a third diameter. The first diameter is smaller than the third diameter but larger than the second diameter. The third diameter is larger than both the first and second diameters. The first part is suitable for engagement with the bearings 140b. The first bearing support 310 comprises a first part, at one end thereof (the end distal from the second bearing support 320) having a fourth diameter, and a second part, at the opposite end thereof having a fifth diameter. The fourth diameter is preferably the same as the first diameter of the second bearing support 320, and is smaller than the fifth diameter. The fifth diameter is preferably the same as the third diameter. Both the first bearing support 310 and the second bearing support 320 are at least partially hollow, with various parts of the measurement tool 300 being disposed therein. A linear bearing 340 is disposed around the second part of the second bearing support 320, and within the second part of the first bearing support 310. This serves as a piloting aid to permit the first bearing support 310 and the second bearing support 320 to move axially with respect to each other without moving out of axial alignment with each other. This reduces the sensitivity of the load measurement tool 300 to side loading. A bolt 370 is used to mount the sensor 360 to the second bearing support 320, and also to pre-load the sensor 360 with a baseline level of compressive load. This is because a load cell generally has an operating range of between a positive preload value of (for example) 50N, and an upper value of (for example 500N). It is therefore desirable to form the tool 300 in a manner which preloads the load cell at or above the minimum operating value of the load cell 360. This initial preload value may be calibrated to zero for measuring the preload on the tool 300 as a whole. The bolt 370 extends from inside the first bearing support 310 to the inside of the second bearing support 320, while passing through a bore in the centre of the load cell 360. A bush 375 is provided around the bolt 370 and within the through bore, for centralising the load cell 360 and keeping it separate from the bolt 370. A spacer 350, which can be swapped out for other spacers having a different thickness, is used to allow for simulating part-to-part variation. That is, the tool 300 can emulate the dimensions of different shafts 200 having slightly different lengths by switching the spacer 350 for a different one. The first bearing support 310 and the second bearing support 320 are formed of steel. A tool housing 330 formed of plastic extends around the third part of the second bearing support 320 and the second part of the first bearing support 310. Part of an exterior surface of the tool housing 330 is externally threaded for engagement with a gear of the electric drive unit I transmission. The housing 330 is attached to the first bearing support 310 and the second bearing support 320 with (grub) screws. In order that the first bearing support 310 and the second bearing support 320 are able to move axially with respect to each other, at least some of the openings in the housing 330 through which the screws extend are shaped and dimensioned (elongated in a direction parallel with the longitudinal axis of the tool 300) to permit the screws to move with respect to the housing 330. This is shown in the right hand portion of Figure 5. The second part of the first bearing support 310 comprises a plurality of longitudinal open channels, and the tool housing 330 comprises a corresponding plurality of openings which line up with the open channels. A light position sensor 380 is provided in at least one of the open channels. Preferably though, light position sensors are provided in multiple of the open channels to be able to gauge tilt I misalignment in the tool (from differences in displacements measured by different ones of the light sensors), which may occur if there is some play in the linear bearing 340. The light position sensor is fixed to or with respect to the first bearing support 310, and measures a distance to part of the second bearing support 320 (in particular to an end face of the second part of the second bearing support 320 which faces the first bearing support 310). With the position sensor, a displacement can be measured (distance from the light sensor, which is mounted on one of the bearing supports, to a fixed point on the other of the bearing supports). This displacement can be correlated with the preload measured with the load cell, and by changing the shim and measuring both again (preferably multiple times) it is possible to determine if the measured preload and the measured displacement have a linear relationship. Such a linear relationship is what would be expected from the real transmission shaft (being a solid shaft), and so the ability to confirm this linear relationship with the tool (and to calibrate the tool if necessary) is beneficial. In addition, by providing the light position sensors in several locations on the tool, it is possible to observe if there is an angle of misalignment on the shaft when the EDU housing is closed and identify whether it has a preferred orientation and "jumps" when the shaft is rotated. Figure 6 schematically illustrates a method of estimating a preload on a transmission shaft of an electric drive unit using a measurement tool 300 as described above. The method comprises a step S1 of providing the second casing part 120 of the electric drive unit 10. Then, at a step S2 a shim is inserted into the second casing part 120, and onto that the cone of the taper bearings 140b is assembled. It will thus be understood that the shim is provided, once assembled, between the second casing part 120 and the bearings 140b. The cone of the taper bearings 140a is similarly installed (generally without a shim) into the first casing part 110. Then, at a step S3 the measurement tool 300 is loaded onto the second casing part 120 via the first set of bearings 140b. Then, at a step S4 one or more wires from an output of the load cell and position sensors are wound around the measurement tool 300 in a first direction and fed out of the casing. At a step S5 the second casing part 120, tool 300 (and other gears installed) are all offered up and abutted to the first casing part 110, and the two casing parts bolted or otherwise secured together, to trap the measurement tool 300 between the first and second casing parts. Then, at a step S6 a preload exerted on the preload measurement tool by the first and second casing parts via the first and second sets of bearings is measured, with the measurement being outputted at a step S7. At the steps S6 and S7 respectively the displacement measurements from the light sensor may also be taken and output. At a step S8, the measurement tool is rotated and the process loops back to the step S6 for another measurement to be taken or output. This additional measurement (and subsequent measurements if the loop of steps S6 and S7 continues) may be taken while the measurement tool 300 is stationary (but at a different rotational position as a result of the step S8), or may be taken while the measurement tool 300 is being rotated. In this way the preload (and corresponding displacement measurement) can be measured at different rotational positions of the measurement tool 300, and while it is in motion. The steps S4 and S8 may be considered optional, since it may be sufficient to measure the preload of the measurement tool 300 while stationary. However, where the steps S4 and S8 are being implemented, the step S8 should cause the measurement tool to rotate in a second direction opposite to the first direction (that is, in the opposite direction to the direction in which the wires were wrapped around the measurement tool at the step S4). In some implementations, shims may be inserted between the second casing part and the measurement tool as well as, or instead of between the first casing part and the measurement tool. The above-described preload tool is designed to be equivalent to the transmission shafts (intermediate / lay shaft and differential) for use in an electric drive unit and can be fitted in place of them. The tool is able to measure the preload exerted on the shafts via the load cell 360 and output wiring (to be routed through the e-machine cavity). The tool is used to gather preload data for a given shim selection, whereupon the data can be correlated with the current shim measurement process which measures the height displacement of the bearing and shaft assembly into one side of the transmission housing created from simulated preload (weight) applied to the bearing. The tool can be reused on many units and with different bearings. The tool can be assembled with different (controlled) sized spacers that will replicate the effect of part-to-part variation in the transmission shafts. The light position sensor, if fitted, can be used so that displacement data can also be gathered from the preload and correlated back to the current process. The tool can be rotated to mimic the real transmission rotations, and to “settle” the bearings and give accurate readings during or after rotations. It will be appreciated that the present technique does not result in a fully functional electric drive unit. Instead, it effectively results in a test unit being constructed which can be expected to be a close approximation of a functional electric drive unit. Utilising the tool requires fully assembling the transmission and housings before testing, and following this the unit is disassembled, and may be reassembled with a different configuration of shims. It will also be appreciated that to the extent that the instrumentation wires are be fed through the E-machine cavity or transmission cavity the testing cannot be completed on a fully assembled unit (only transmission assembled). However, following the testing process it is possible to readily remove the tool and bearings and rebuild the unit with proper parts and take efficiency measurements from that unit. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A load measurement tool, comprising:a first bearing support, for engagement with a first set of bearings;a second bearing support, for engagement with a second set of bearings;a sensor, mounted with respect to the first and second bearing supports and configured to output a signal indicative of a load exerted on the load measurement tool.

2. The load measurement tool according to claim 1, wherein the sensor is a load cell, which in use is compressed between the first bearing support and the second bearing support, the signal output from the load cell being indicative of an axial force exerted on the load measurement tool via the first and second sets of bearings.

3. The load measurement tool according to claim 1 or claim 2, wherein the first set of bearings is mounted between the first bearing support and a first part of an electric drive unit, and the second set of bearings is mounted between the second bearing support and a second part of the electric drive unit.

4. The load measurement tool according to any preceding claim, wherein the first part of the electric drive unit is a first casing part of the electric drive unit and the second part of the electric drive unit is a second casing part of the electric drive unit, and wherein the load measurement tool is configured to be braced between the first casing part and the second casing part via the first set of bearings and the second set of bearings.

5. The load measurement tool according to any preceding claim, wherein the first bearing support and the second bearing support are coupled together, and the load cell is mounted between a portion of the first bearing support and a portion of the second bearing support.

6. The load measurement tool according to any preceding claim comprising a housing disposed around at least a portion of the first bearing support and the second bearing support.

7. The load measurement tool of claim 6, wherein the housing comprises an external threading for engagement with a gear of a transmission.

8. The load measurement tool of claim 3, comprising a position sensor, for measuring a displacement from a fixed position with respect to the first bearing support to a fixed position with respect to the second bearing support.

9. The load measurement tool of claim 8, wherein the position sensor is a light sensor.

10. The load measurement tool according to any preceding claim, wherein the tool has a shape andstiffness representative of a transmission shaft.

11. The load measurement tool according to any preceding claim, wherein the first set of bearings and / or the second set of bearings are taper roller bearings.

12. A method of estimating a load on a transmission shaft of an electric drive unit using a measurement tool according to any preceding claim, comprising the steps of:providing a second casing part of the electric drive unit;mounting the measurement tool to the second casing part via the second set of bearings;mounting a first casing part onto the second casing part, to trap the measurement tool between the first and second casing parts; andmeasuring a load exerted on the load measurement tool by the first and second casing parts via the first and second sets of bearings.

13. The method according to claim 12, comprising a step of inserting a shim between one or both of the first casing part and the first set of bearings, and the second casing part and the second set of bearings.

14. The method according to claim 12 or claim 13, comprising causing the measurement tool to rotate during the measurement step, or between subsequent measurement steps.

15. The method according to claim 14, comprising winding one or more wires from an output of theload cell around the measurement tool in a first direction prior to the measurement step, and wherein the step of causing the measurement tool to rotate comprises rotating the measurement tool in a second direction opposite of the first direction.s

Images